I was shopping around for a cheap 5.1 receiver last month, and I honestly couldn't find anything THAT cheap. Best I could find was the Sony DG510 for $150. Ended up buying the DG710 for $200 shipped on Amazon though.
If that's out of your price range, you might be best off getting one of those Logitech 5.1 speaker set ups. I don't think $100 could even buy a 2.0 stereo receiver.