Two honest questions this answers: should you alch or sell an item you already own? and what does training Magic by alching actually cost? Pick an item to see its live high-alch value against its Grand Exchange price, per cast and per hour.
Ranked by realistic profit per 4-hour buy limit — because the buy limit, not the per-cast margin, is what caps the gold. 132 items currently profit to alch.
| # | Item | Buy price | High alch | Profit / cast | Limit / 4h | Max / 4h |
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Compare its high-alch value to its Grand Exchange price. If the high-alch value (minus the 132 gp nature rune) is higher than what you'd get selling it, alch it; otherwise sell. For almost every item players actually want, selling wins — the calculator shows the verdict for any item.
Not really as a standalone money method. Buying items just to alch them is capped by each item's 4-hour Grand Exchange buy limit, so the gold trickles in slowly. High alch is best thought of as a way to train Magic that roughly pays for itself (or turns a small profit) on items you already have.
Each High Alchemy cast gives 65 Magic XP. At roughly 1,200 casts per hour that's about 78,000 XP/hour, whichever item you alch. The gold per hour depends on the item you pick.
An item's high-alch value is a fixed fraction of its shop value, but its Grand Exchange price floats with demand. For anything players want, the market price sits above the alch value — so alching would destroy gold. Only items whose demand has fallen below their alch value turn a profit.