Where's a good place
I saw a neo geo cd and a cart system at one time in one of the local game stores used for $60 apiece. I think it was an electronics boutique. I wish I would have gotten them at the time but couldn't really afford it when I saw it. The best price you'll find is usually going to be in Asia, but the shipping price is high if it's going to North America. I've seen them go for around $80 sometimes from Japan, but insured shipping could be up to $80 itself for one.
Just keep an eye on EBAY, and you should be able to find one for around $200 or less shipped.