Does anyone know how big of an ordeal and how expensive it is to register a mature mare if I can get together all the paperwork?

Here is what you need to know both parents have to have DNA on file. Dam must be on the stallion report from stallion owner. You must get a breeder's certificate from stallion owner. Your mare has to be registered in the name if the owner at the time of breeding. This also requires an AQHA membership or the registration fee is $65 dollars higher. Once you get the mare registered, you have to have a signed transfer from that owner to you. You will have to pay for transfer and AQHA membership or pay $65 more for transfer. Registration of a horse over 4 is about $500. People post all the time that they get DNA on a horse and AQHA just sends them the papers so you might try that. I have a terrible time getting my babies registered and my paperwork is always in order. I have a brand new AQHA hand book if you want a list of all the fees.