Builders want planks. You start with logs. Check the cost of the step between them. A mahogany log and a mahogany plank are not the same product. Turning one into the other can pay. But the sawmill takes gold. The spell takes gold and runes. Count both before you start.
OSRS-GE is a free Old School RuneScape Grand Exchange tracker. Prices come from the OSRS Wiki Real-time Prices API and refresh every minute. Every margin shown subtracts the 2% GE tax, capped at 5m gp per item, and respects each item's 4-hour buy limit, so you see the profit you can actually make. Use the calculators to compare flipping, overnight offers, High Alchemy, herb cleaning, decanting, Plank Make, tanning and other money making methods, and read the guides to learn margin checking, how to avoid fake margins and when to trade.