Her worth is what she can do, health status, and your location. While older horses may require medication in order to continue be rideable, they can continue to serve the people who own them. If you are interested in purchasing this horse for your own use, have a prepurchase exam on the mare. If you are simply planning to flip her, you'll have to check your local craigslists, newspapers and see what similar aged horses are selling for. Around here a proven child safe horse at that age in good condition sells for around 1,000 bucks. One that has a talent such as barrel racing might sell for 2,000 or so.