It varies so widely that I don't think anyone could answer that question. If you are wanting to buy locally instead of online then I would suggest looking at any local feed or tack stores, Craigslist, and asking around to see if you can find someone trying to sell things. Once you find something, look online to see if the price and quality seems reasonable. Sometimes you'll find people selling stuff for cheap to get rid of it, and sometimes you'll find stuff with the price marked up drastically- especially if you're looking locally.
Beyond that, it depends on the brand and quality of the tack you're looking at. For example, you can find a cheapo Indian-made saddle for $100. You can also find a very nice saddle for multiple thousands. If you're new to the horse world then I would recommend looking into brands that sell used starting at a few hundred. You can get a good deal on some saddles for less than that if you know what to look for, but if you don't then you can wind up with a piece of junk that will fall apart and hurt your horse's back. When you find something that you're interested in I would recommend doing a search on Google and on this forum to see what other folks think about the brand and style of saddle you're looking to buy.