Found this article in my notes from…err 4 years ago. Never got around to posting it till now.
Its a nice simple build for a rain barrel. Only concern would be BPA issues, but if you are just watering the lawn and stuff, Should be fine. The author puts the cost at under a $100, but I bet with scraps around your house, you could do it for under $50.
http://www.familyhandyman.com/smart-homeowner/how-to-build-a-rain-barrel/view-all