Another source for cheap bikes is police auctions. Police often accumulate so many unclaimed lost or stolen bikes, watches, glasses, dentures, ... that they need to auction them off. Here in Trondheim I think the auctions are held twice a year. I think you're allowed to go look at the items before the auction to see what you want to bid on ... or if your own stolen bike is among them (last chance to claim it). Not all that convenient with only 2 shopping days per year, but you can get some good bargains if you have the time on the day of the event.