My computer system uses a pair of Boston Acoustic Bravo's picked up on clearance for Circuit City (remember them?) and powered by a Sony HT receiver. They're wall mounted with an old Sony Sub to fill in the bottom end. Typically they run between $150 and $200 each, probably out of your price range. Another possibility would be the BA Soundware cube speakers, which run between $50 and $100 each, again needing a sub. They are small and cute with decent sound and also wall mountable.
The Pioneer SP-BS41-LRs may be the best bet at this price, but are rather large and rear ported which limits your placement options. However, you could get by without a sub with them. Thrift shops may have some good options, but can be difficult if you're looking with specific needs. There are a lot of old computer speaker systems that can be found there, but sound quality is typically a cut below most of the items mentioned in this thread. Price is usually < $30 so may be a good stop gap.