Old Collegiate saddles (the older AP; the Alumnus, I think), are good on your budget. Wouldn't buy a new Collegiate, the leather quality is not as good. Henri de Rivel makes a nice saddle too, you can sometimes find them on sale, I found it to be a very comfortable saddle. Stubben makes a nice AP (you'd have to find a used one to fit your budget). Overall, I'd go used unless you find a new saddle that you really love; you can get so much quality in a well-kept used saddle and its nice that they've already been broken in for you:P
The hardest part is finding a saddle that fits your horse or (even harder) if you are trying to buy a saddle to fit a variety of horses. While all of the saddles I listed where comfy to ME, only the Collegiate was comfy to the 4 horses I was riding at the time I bought it.