That my dear, will TOTALLY depend upon the saddle. IMO you get what you pay for. I'd look for brand names in this case. BUT...This is where it get's tricky, Some custom saddles will be made by names you have never heard of. A custom western saddle can cost in the neighborhood of $3000 easily. So don't have too much sticker shock when you see a used saddle for $1500...that could be a screaming deal...OR a screaming scam.
I personally like Tex Tan, Circle Y, Billy Cook (Oklahoma built), Dakota saddlery is a less expensive brand that Bill (Iride) has mentioned several times and I have seen a few in teh stores and I am impressed with the quality Vs. Price. Tho I have never ridden in one.
You will want to take a tape measure!!! Not all seats are the same! You'll also want to learn how to measure the gullet and actually measure it before you buy. Don't take anyone's word on what they have...they're selling it and you need to be a smart shopper.
I like to look for roping saddles. They are usually built to last and with stand the rigors of actually roping. I like barrel saddles tho too. Nothing wrong with a ranch saddle or trail saddle either.
Read this site...top to bottom!!!! And all the pages...You'll want to be armed with as much info as possible!!!! Western Saddle Seat Size: What Size Am I?
Google what to look for when buying a used saddle. You'll want to know how to check the tree and check the riggin'.
Now...the nitty gritty price detail. Most good saddles (of the brands I posted) run in the neighborhood of $1200 + or - (except the Dakota saddles...they run about half that). TOTALLY depending on what kind/make/model/age and wear you can expect to pay anywhere from $1000 to as little as $200. It's a buyer's market now IMHO. There are lots of saddles on the market as people are selling off their herds. Especially heading into winter. BE CHOOSEY!!! It's ok to go home without a saddle. Get someone's number if you just aren't sure. IF they still have the saddle after the sale chances are they might take less.
Good Luck Moxie!!!