you can get them from about $500 to $2k+.
I've got a cheapish 12' Ascend sit on top that I love, nice and wide and stable, got it from Bass Pro. Hobie is a pretty high end brand, but there are a lot of options to pick from.
Lots of info here and on other internet resources, it's a great way to fish.
http://www.kayakfishmag.com/boats/