A good used western saddle will be in the $500 - 750 range (only luck will find a good saddle in the $300 range) although you can get a nicely make, made in America saddle new in the $800 to 1,000 range if you look at Corriente, or Dakota.
For just trail riding and/or some arena work, I would stick to a pleasure saddle, or a ranch saddle. Personally, I would not consider a barrel or reining saddle although many people use them. Ebay
can be a good source of used saddles but many people can be taken if they don't know enough about saddles or makers. As far as trees, that can be a tricky thing since there is no set rules as to tree sizes. What one maker considers a QH tree, another may see it as a semi QH. Your TB many work better with an A fork saddle with semi QH bars rather then a Wade due to the high withers.
I would suggest trying a few saddles from your tack shop to see what works for him. There is also a thread http://www.horseforum.com/horse-tack...dle-fit-45868/
that may help you.
In any case, if you see a saddle on ebay, come on here and ask about it. You may find just what you are looking for or find one that you are warned to stay away from.