I got two of my tanks used on craigslist.com I got my 30 gallon with filter, hood, light, heater, air pump, stand, gravel, lots of plants and various other accessories for $50. I thought it was a great deal.
I honestly can't remember what I paid for my 29 gallon setup. I would look at several local pet shops as well as the chains before I made my decision. For example, I buy all my tanks at one shop about 50 miles from my house because they have deals that you can't beat anywhere else in the area.
$1.00 per gallon is a good general rule of thumb when it comes to used tanks on something like Craigslist. Be picky and wary about used though.
I agree with Courtney 8526. I am pretty new here and have just been reading alot, but I have been searching Craigslist for the past 2 weeks and just purchased a great system dirt cheap. I would check there or other classifieds in your area. There is always plenty out there because of moves,etc. Good luck!