How old is she? The cost to register with the AQHA goes up significantly depending on the horse's age. Google the AQHA website and search for registration fees. My mare can be registered, but she isn't, and at 3 years of age it'd cost me almost $600 to register her at this point....it's ridiculous. I'd say $200 isn't bad to register the mare you are considering tho but you better check with the AQHA first.
Scroll down where it says registration.... http://www.aqha.com/forms/index.html