When you're considering a band and they seem unusually inexpensive, check to see if they're planning to provide their own sound system. If not, you'll be stuck with having to provide a sound system of your own, which can run up the cost excessively.
Most bands don't come cheap. The least you should expect to pay for a wedding or party band should be about $500; the average price is more in the range of $1,000-1,500. Of course, if you want a national act to play at your event, expect to pay $50,000+.