Craigslist will likely be cheaper than eBay, because eBay is a much larger market. Also, with Craigslist, there's no shipping charge involved. And as somebody pointed out above, you have to be cautious on eBay, because some sellers will charge a flat fee for shipping, some charge the actual cost, and some charge an exorbitant shipping charge. ALWAYS make sure you know what the shipping charge is before bidding on eBay (or at least get a commitment from the buyer that it will be actual cost), and figure that into the total cost for your bike, otherwise, you may find the seller charging well over $100 for shipping an item that shouldn't cost more than $50-$75.

Another good option is yard sales/garage sales. Lots of people here find bikes cheap that way, then turn around and sell them on Craigslist or eBay for what they're really worth. Thrift stores are another good source, if there's nobody in your town picking through them already.

One thing, when you find a bike you want at a yard sale or thrift store, at a bargain price, you've got to move fast, or it will be gone by the time you make your mind up. That means knowing what to look for before you go looking.