I believe the price varies depending on the difficulty of the case ie the number of stamps in the passport etc so they look through the passport and then give a price. He can't a Non-O based on a Thai child until he has legal rights which can take many months
I suspect you are not on a visa at all but rather you are on an extension of stay, take a look in your passport and check. If you are on an extension of stay it is not void because you travelled out of Thailand, you need to get paperwork from the school you studied at and go and cancel it at the relevant immigration office in Thailand. It shouldn't stop you entering but can be a real issue going forward
Have you got a visa or an extension of stay. If its a visa just leave it as you need it for the extension. If it's an extension of stay I'd leave until after the under consideration period then you can do what you want with it, but given you will have already changed it you may as well use it for spending money