A few weeks ago, Media Markt (German equivalent of Best Buy) was selling unlocked Sony Ericsson Xperia-somethings for only 29EUR. Granted, it was basic, but it did send SMS and surf the web. If what you want is temporary, this might be a good option.

But yes, as Oliver suggests, the non-descript handy shops will be good for a cheap phone, but if you're like me, you want something brand new just so you know you shouldn't have any issues, and can always take it back if so.

Good luck!