You can earn quite a decent sum of money from selling strength potions. Each dose of this potion is worth 250gps, totaling an amount of 1k for each full vial of strength
potion (4 doses in a full vial).

The process for obtaining a strength potion is fairly simple. In order to enchange for one, bring the Apothecary in Varrock (the apothecary is located in a building with a
water source south of Varrock’s West Bank) a Limpwurt Root and a Red Spider Egg,along with 5gps. Ask him to make you a Strength potion and follow through answering his
comments. Regular players can obtain up to 25k per hour.

How to obtain Limpwurt Roots:
Limpwurt Roots are dropped by several creatures. Acquire a full inventory of these
along with the full inventory of Red Spider Eggs before depositing them in the bank for the quickest and most efficient method of making money this way. Below is a short list of some creatures that drop Limpwurt Roots.

How to obtain Red Spider Eggs:

There are several locations to acquire Red Spider Eggs (mostly in caverns and dungeons). One is in the Varrock Dunegon. There is a spawning point of this egg at the
end of the passageway. It takes several moments for this egg to respawn. Be careful
when coming here because you’ll have to elude through several high-level creatures and
the chamber where the eggs respawn at is full of Red Spiders. They’ll attack anyone who comes within their line of sight with a combat level of 50 or lower.

The other location for Red Spider Eggs is in the Karamja Dungeon. There is a spawn point of this where the Red Spiders are south of the odd-looking wall. You can access down the ladder east of Brimhaven on the island of Karamja for the closest possible entrance to this spawning point. Accumulate an inventory full of these before depositing them at the bank to make for good time.