Buying at the right price

Question
I want to make sure I don't overpay if I purchase a horse I test rode yesterday.

She's a 14 year old 16.1 hand thoroughbred dappled brown mare located in New Jersey. Very good looking and a 1 temperment from what I saw (I'm not an expert on conformation, so couldn't comment).

She was a nice round jumper, but jumps only  2.6-2.9 and too slow for her teenaged owner, who needs to compete (says she did ok at the hunters but not jumpers, where speed is a factor). Nicely trained: Does her flying lead changes with ease; gives a very safe steady ride; and knows all the aids. She had  a cushy canter and the trot was even easy to sit.

If there was a negative, I would have to say she lacks a little go. But I'm 45 and a beginner/intermediate and really liked the feel of her movements and her responsiveness. Also seemed to need the perfect contact with the bit (too little and she'd slow down, too much & she'd squirm). Nonetheless, pretty, dappled brown & never lame.

Still  I thought she was priced way too high.  The trainer who brought me to her said they were asking $18,000 & I don't really believe it -- I had called about a $20,000 6 year old, which was totally wrong for me -- and she would up showing me two others, both amazingly near to $20,000 mark.  Where would you price her?

Answer
$18-20k is perfectly reasonable for a seasoned show hunter.  Here is what I factor in to buying a horse:
 Health - have a vet check the horse for a clean bill of health before you purchase...ALWAYS!
 Training - is the horse trained for my needs?  If not, what will the cost of training be to prepare the horse for my needs?
 Age - Is the horse at an age suitable for my needs and/or growth as a rider?
 Safe - is the horse safe for my abilities as a rider?
 Location - Is the horse located nearby?  How much would it cost to transport the horse to my stable?
 Personality - Did I enjoy working with the horse?  Did he/she enjoy me?
 Athletic Ability - Can the horse perform my demands/needs?
 Reputation - What is the horse known for?  Is the horse known for reliable and good behavior or bad behavior?
 Ground Manners - Is the horse displaying a good temperament?  Is his temperament something I can improve if not entirely desirable?
 Results - If am competing with the horse, could he help bring results?  Is he a good mover?  Jumper?
All of these things factor into purchasing a horse for me.  If the price of the horse seems too high for my budget or too high compared to the answers of the above questions, then I move on.  If not, then I buy.