Why do you want new wheels? What's wrong with the existing set?

Normally, people buying new wheels are looking for enhanced performance, and so the market is weighted towards better high-end wheels. People who don't care so much about performance will more than likely just stick with the OEM wheels that came with their bike.

For road bikes, entry level wheels would be something like Mavic Aksiums or maybe Shimano RS20s, which run about $250 for the set. But again, it's hard to answer the question without knowing why you are looking to upgrade.