I have to agree with what others have said, for $30 you are not going to find much decent. Sense you mention wanting bass probably any 2.1 system will do. Just find a few under $30 and check which one gets the best reviews on Amazon.
If you did not mention wanting bass I would suggest looking for a pair of USB powered speakers. I had a pair of Logitech Z-5 (now discontinued) before I got my Audioengine A2 that I got for $30 and was actually quite pleased with how they sounded for the price. I believe it was in large part due to being USB speakers, which allowed them to bypass the poor soundcard on my laptop. But sense you mention bass I think a cheap 2.1 setup with a sub would be your best bet.