You want to get a mount at 40 but 90gold seems like a kingly sum. Well, if you’re just playing normally and grinding, odds are you will be lacking the cash at 40. If you make an effort to collect certain items however, you will be richly rewarded.
Professions
I highly suggest taking mining and herbalism. You really don’t need trade skills if you’re in a guild and if by chance you want to pick one up later, you can always drop one collection skill for a fabricating one (engineering or leatherworking for example).
Going the mining route you will want to hunt near mountains and hills. You won’t find too many mineral deposits in the open fields. Best bet is to hunt in caves. 90% of the time you will be fighting humanoids, the spawn is easy to read and there’s ton of minerals to collect. Just collecting it through you leveling should be enough. The Alterac Mountains in your 30s is a good place to mine, or mining the southern wall of Duskwood. In the 40s the Badlands are great to mine as you’ll nearly always have an available mine on your map. Tanaris is next on the list. Ungoro Crater is great once you hit 50 as well. Remember to fight along the mountain side (usually the zone borders) and you’ll be set. It also makes fighting safer, since you don’t have to watch your back as much. Mines are heavily spread out in the Outlands but Hellfire, Nagrand and Blade’s Edge Mountain are your best bets.
Herbalism is my 2ndnd choice because of the amount of herbs consumed making potions. Since most server have a high population of 70s, potions are used all the time. Collecting herbs can make a tidy sum of money and you’re not going out of your way to collect anything. It’s usually exactly where you’re hunting.

Some suggest that Enchanting is a good trade to make money from. Now, since the goal is to get to 100gold before level 40, I can assure you that Enchanting will not do that. The costs are quite high to increase skill gain and you’re required to disenchant “green or better” items for parts. Instead of selling those items for cash, you break them down.
From personal experience, I would avoid trying to make money from any skill that makes final products. It takes time and money to make an item and usually for only a few silver more than you made it for. Collection skills (mining, skinning and herbalism) are what people need to keep supplied. People don’t replace their items all the time but crafters continuously need supplies to create new items.
Once you reach 70, Enchanting and Jewelcrafting currently make the most money. Expect to spend upwards of 1000g on each to train them however.
Looting
Humanoids have the best chance of dropping items bar none. Sure, we’ve all found cool items on beasts but the chances are a lot lower. If you’re grinding off humanoid casters you will find a lot of green items drop. Rule of thumb, if you see an item with a white name drop and it’s stackable, keep it. Stacks of 20 can really help out.
In Stranglethorn Vale (STV) there are trolls that drop Flasks of Mojo. Keep these and sell them on the Auction House. They sell for about 1gold for 4. Also, you will be fighting goblins in the Venture Co. Basecamp. Keep the Wildvine that drops!!

Once in the 40s, there are some interesting areas to fight. In Tanaris, there are bandits that have a high chance to drop Scorpid Leather patterns as well as Mageweave. They also have a perpetual quest to collect water skins, which you should be doing to get extra experience. In the mid 40s, head to The Hinterlands and the Altar of Zul. Killing only the Soothsayers is really easy experience and similar to the bandits, there is a troll necklace quest that gives 390xp per 5 collected. There’s a very good chance of greens dropping and a rare troll pops from time to time, Retherokk the Berserker, who always drops a green item. Some people will say the Tanaris ogres are better. Ignore them and do the trolls. To me, this is the best place to make money in the entire game. Wildvine

drops from this location and can sell for 15g for stacks of 20 with little effort. If you grind from 45-50 here, you can leave with a few hundred gold from items and wildvine.
In the 50s, money just seems to be rolling in from everything you kill. Grey items sell for above 1gold to a vendor, so keep every weapon you loot. Again, humanoids tend to have the highest drop rates for weapons in the game. Sure, Demons/Undead can drop items but no where close (in my experience) to true humanoids. Once you hit 60, I suggest killing level 55 humanoids for their items. They are easy kills and you won’t need to heal much as you will overpower them. At 60, you can hunt in Tyr’s Hand. The humanoids are elite but rather weak. They regularly drop 20s per kill and since they are elite, they have a high chance of dropping green/blues compared to other creatures. The downside is that there are a lot of people here getting cash too.
From 60-70 there are a few general rules to abide.
●By/make/get 16 slot bags
●Keep all “Motes”. Shadow, Water, Fire, Earth, Air, Mana. You can combine these into Primals which sell for 20-30g depending on type and your server.
●Stacks of 20 grey items sell for about 2g per stack.
●White Items (ie Netherweb Silk) can sell for some pretty crazy prices depending on your server. The Silk on mine sells for 10g and has a 20% drop rate.
●When selecting a Quest Reward and you can’t use any, take the Plate option or the 2-hand weapon option. Both tend to sell for more.
Gereral rules for maximizing gold gain.
●Keep your bags organized and with very few items. Some people claim to greatly increase drop rates when their bags are low.
●Hunt Humanoids that drop items. By this, I mean they regularly drop something as well as cash. It could be food, grey items, what not. These are usually placeholders for other items. Personally, I aim for creatures that drop at least 1 item every other kill.
●Stay out of instances. Most of the time, you need to group to get anything decent. If you feel able to stealth into an instance and kill something solo, by all means try it out. Grouping reduces your cash intake because you have to split it 5 ways.

●Using the AH, use the suggested starting price as a gauge to set the buy-out. If the item starts at 1g, then have a 5g buyout.
That being said, know what is actually useful. A cloth item with agility and strength is completely uselss to cloth wearers. Pay attention to the stats. Here’s a quick list of what classes look for.
Warrior – Plate with Str + Sta
Paladin – Plate with Str + Sta
Hunter – Leather/Mail with Agi + Sta
Druid – Leather with Sta + Agi/Str/Int (this depends on their talent selection)
Mage – Cloth with Int + Sta/Wis
Warlock – Cloth with Int + Sta/Wis
Priest – Cloth with Int + Sta/Wis
Shaman – Mail with Int + Sta/Str/Wis
Keep all stackable “white” items. They generally sell for a lot of cash.
If you spend a play session (3-4 hours) simply hunting for items, you can expect to get 3-4 green items drop, maybe even more. I also highly suggest checking out www.thottbot.com for certain items and see where items that are world drops are more commonly found. The Krol Blade for example, is found in the Plaguelands and Ghosts in Winterspring. Concentrate on those enemies for maximum gain.
Netherweave Silk currently drops from nearly all humanoids 62+. Each stack can get you 5g or more. Some people farm elementals for Motes, also an effective way of making cash. Netherweb Silk is by far the easiest way to make money, if you have the ability to cleanse poisons and heal. It drops every 4-5 kills in Terokkar Forest, just south of Veil Shalas and can sell for 10g per.
Questing
Quests give cash and item rewards. From 1-40 questing can be a good way to supplement your income; the quest in Stranglethorn Valley are a good example. Once you reach the Outlands however, things change a bit. If you do the solo quests as you can from 60-70, you are guaranteed to amass at least 1000 gold. Once you hit 70, any quests left over will give you more cash as you can’t gain experience anymore. So a quest that would give 8g at 69 would give 20g at 70.
Without much effort I have made over 3500 gold from 60-70 combining quests, dropped loot and random world drops.