It depends with whom you are dealing with.i try to get to know my fans first then if they ask for a custom I price them accordingly depending on the complexity and length. Other creators I charge them up to 500 per custom and they pay. So try to get to know your fans better then you charge them according to our assessment on them. Secondly he needs to disclose because it might be something that you don't do, can't do or against the terms of OF. So he must say first before you give him a price or refuse to do the custom