The prices will probably be going up now as it is spring, but honestly for a pet mini I think that you could find a nice, calm, pet quality one for a lot less. Their price might be quite reasonable for what is being offered, though. Obviously the price will reflect factors like their success in showing, their level of training, whether they are a good breeding animal, etc. If you don't care about breeding or showing, you may not want to "pay extra" for these things.
Another thing to consider is that many people are careful not to price their horses too low because they want to screen out killer buyers and people who haven't given adequate thought to what they are about to undertake. If you care about your horse, you don't want someone reading the ad and thinking "hey, this one is only $250, so if it doesn't work out, no big deal, and I can just sell it at the auction..." You might find that once you meet the horse and talk with the owner that if you can show that you are a responsible person offering a lifetime home, they might be willing to let the horse go for a lot less than the asking price.