How much would this kind of horse cost?

I'm wanting to get my first horse, and I'd really like to get some type of paint mare or gelding (doesn't matter). I also want to get an older one that's been trained. Better than an inexperienced horse, that's for sure. Somewhere around 15 years old, if possible. I have pasture/barn, equipment, and food all worked out. Just wondering how much I'd have to pay for one that trained and possibly around 300 dollars. (sorry if I'm wrong about that cost, so please don't be rude if I am). Thanks!