|Tier 20+ two handed magic or ranged weapon|
190 × Grapes (1,292,570)
Guards are found in various locations around Gielinor, with six of them located at the Varrock Palace. On death, they occasionally drop 3 grapes, used for making wine and each worth 6,803. Without competition, this is a good way to earn money in free-to-play.
Teleport to Varrock using the lodestone, and head north towards the palace. Attack the guards using whatever style is best, although they are weak to air spells. Each guard takes a little less than 60 seconds to respawn, and you should be able to kill all six easily within that period. At higher combat levels, it's also possible to cycle between two worlds using the "Join" function on the Friends List, letting you kill about twice as many guards per hour. The guards drop three grapes about 20% of the time. When your inventory is full, head to the bank located just south west of the palace. The automatic healing inside the bank will negate any need for food. When you're finished, sell the grapes on the Grand Exchange.