A deposit is not a profit centre — it is a risk buffer. The right amount is tied to what it would cost you to repair or replace the item, and to how likely that is for this particular customer.
Start from replacement value, not rental price. A camera that rents for 40 a day may cost 2,000 to replace; a folding chair that rents for 2 costs 15. The deposit should scale with the downside, not the income.
Then adjust for evidence. A documented condition record at pickup — photos, meter readings, a signed checklist — lets you charge confidently for real damage, which means you can hold a smaller deposit without carrying more risk. Good records are cheaper than big deposits.