In this market, the going rate for a off-track Thoroughbred not retrained for anything in particular is $200-$500 so that wouldn't supprise me.
Secondly, assume she bowed a tendon at some point, or there is some reason she was retired unless she retired young from just being slow. Thats a given, most are still great horses.
Also, just understand if you plan to ride her outside (trails, roads, fields) she is an off track Thoroughbred and a great trail horse canidate after shes been trained and seen these new things. She is probably not a get on and go horse yet, unless there has been some retraining.
That said, I love the OTTB's :)