In Old School RuneScape, High Level Alchemy is a spell which converts items into money. How can I figure out the best items on which to cast this spell if I want to gain Magic experience? Here, "best" means a combination of low loss per cast (self-explanatory), low unit cost per item (so I can buy a lot of casts' worth of supplies at once), and generous four-hour Grand Exchange buy limit (same reason).
(This answer to a similar question provides an answer for standard RuneScape, i.e. RuneScape 3, in the form of a dynamically-updating website which lists the necessary information.)