I bought my horse (OTTB) when she was 9 or 10, she has pretty good bloodlines and made quiet a bit of money. i bought her for $240.00 Canadian. she is an amazing horse, learnt everything i know on hwe. i cant believe a 23 year old would be too too much, most likely under $1000.
It all depends on his training and experience. He would be a lot more expensive if he was an experienced show horse with many years of training, and he would be a lot cheaper if he did not have much training.