I demoed a whole bunch of speaker sets at microcenter one day in late 2009 in their speaker section. Microcenter has a much larger speaker demo area than most places like Best Buy. I had the same intent as you do - to buy a 2.1 setup for about $50 bucks. My clear conclusion was this set.
Altec Lansing VS2621
To my ears nothing else for around $50 or less compared. Of course I've heard better (I'm really into home theater equipment, and have a pretty nice setup including the recent purchase of a couple of SVS PB13-Ultras that really shine on bass ) - but I've not heard better at $50. At the time they were on sale for $35 at Amazon and qualified for free shipping which made it even more of a win. They don't distort and sound surprisingly good for a cheap set of speakers (they have a high pass cutoff on the sub that makes it cleaner than many of the other cheap sets) yet they seem to get plenty loud for computer use and can easily fill a small office with sound. My wife turns them on sometimes and listens to them in the kitchen - two rooms away - so they do produce reasonable volume. They don't even attempt at the real low LFE stuff - but on a 4" woofer it's better for that size sub to not even try - instead what you get is good music, game and movie playback at good spl levels with no distortion.
If you want pc speakers cheap, I agree with the Altec Lansing VS2621 rec.
One of the reviews cracked me up a bit though:
just got these today,easy hook up BUT i cant belive the amazing sound they produce,you would expect this sound quality frombose,the price was just right, would STRONGLY suggest these pc speaker's for anyone,i am just more than happy with this product,bass,mid's & treble are excellent,cant go wrong with these at all..