okay so l was wondering where is the cheapest place you can rent a surfboard in hawaii? (oahu)
( how much would it cost )

The prices just to rent boards (no lesson) are all about the same. There is a discount, if you can pass as a resident. There are quite a few stands on Waikiki Beach (assuming you will be surfing in Waikiki). There are quite a few used surfboard shops along Kapahulu Ave (one of the inlet roads into Waikiki).

If you already now how to surf and are planning to surf quite a bit, l would recommend you purchase a used board from a shop and sell it back before you leave.

you can go to the waikiki beach and rent them really cheap there. every where really they are not that expencive to rent

assuming you already know how to surf you might also think about renting for multiple days. this can lower the price and you do not have to worry about daily check in/outs plus you can try different spots on the island.