I've seen atleast 3 on craigslist around here recently. sunfish types, sometimes unknown types (but meant for small dinghies, maybe even a windsurfer?), in the range of $100-250.
No idea what they would cost new. I'd just keep an eye on craigslist and ebay and start asking around. You'll find a good deal eventually.
You can also consider
buying a complete
new boat of similar size, maybe in bad shape, for a similar
price.