They are mostly commodity items, one's about the same as the next, exceptions noted below. Go to Goodwill, under $10, usually half that.
I picked up an old 1950's Sunbeam slow cooker/deep fat fryer for $7 a few years ago. The crock is removable so that you can wash it well w/o getting the electrics wet. It has adjustable temperature - probably the best thing about it - so I can make yogurt, slow cook a pork roast, or crank it up for frying.