I've bought many horses from auctions, both riding/driving sales and the kill pen. All the horses are sound and safe enough for anyone to ride. You can find very nice horses there, you just have to know what to look for.
Prices do vary, but I've never bought anything over $500.
Last time I was at an auction was in April, and the most of the kill horses were more expensive than the riding horses.
Look for horses that are calm, but alert. If you find their owner, ask them questions, but take everything they say with a grain of salt. Check for yourself how old they are (look at their teeth), pick their feet up, see if he'll even let you touch his hind legs, see if the horse will let you lean on his back or better yet sit on him. If he seems very nervous, fidgety, and uncomfortable with you on his back, then move on. Chances are he's not broke, or still very very green. If the horse has any scars, ask about them.
You'll want to get there extra early, so you get a chance to see and check out all the horses you're interested in. After you're done looking around, park yourself where the horses are being sold, and keep an eye out for your favorites. Your entire decision on a horse can change when it's being lead/ridden in the ring.