I bought my first one for $175(unit, controller, svideo cable and power supply) from a pawn shop listing on Craigslist when talks of a SD cartridge was first being talked about and then got my second one when I found that B & C Computers (www.myatari.com) still had them as refurbished in the box for $195. looks like new with everything included.
Email them and see if they are still available.