I would keep an eye on ebay for a little while, something always comes up. I've purchased two tanks so far on ebay, one was $35 with a regulator, the other $50 without regulator (aluminum tank). Needle valve and adapter was around $20 with shipping. So if you have the patience to wait for a good deal, you'll certainly find one. I fell asleep waiting for a recent co2 setup to end and it was sold for $55 complete with diffuser and bubble counter.
My local fire extinguisher store also sells new and used cylinders quite cheap and a regulator can be purchased for around $15-20 on ebay.
Hope that helps