Look for a quality used saddle. If you bought a new one for about $300, it won't be the greatest quality. Spending that much time in your saddle during races, you're going to want a quality saddle. I don't know much about specific brands on endurance saddles though. Sorry.
There are no quality leather saddles you can buy new for $300. You will either have to go with an Abetta synthetic or go used. Even Fabtron or Big Horn (also good brands) synthetic saddles are over $300.
To buy enough raw leather to build a western style saddle, it would cost $300 or more, depending on how nice of leather you wanted to build the saddle with. Add to that the saddle tree, hardware, fleece for the underside, thread for stitching, and the saddle maker's time, and you'll understand why a good saddle is going to cost more than your average backyard/trail horse in this market.