I agree, watch out for those cheap e-bay wireless mics. If you go to a music store, just the mic can cost over 700, then the receiver is another 1k. So obviously there is a big difference in quality.
Ah the grand fallicy.
"Greater cost always equals greater quality."
I've seen mics like that at Guitar Center. The ones I saw where $600 for the mic.
The sales person told me it was more expensive because it connected to a digital reciever - which connected to a specialized multi-instrument/mic server system - which is designed to be used with stadium level audio equipment. The set-up needed to use the mic to it's optimum level would cost around $100,000.
Assuming that the system you connect it to is capable of working with it at all:
Will the more expensive mic be clearer? Probably, but if sound quality is your main concern as a magician, you need to rethink your priorities.
Will the more expensive mic work at a greater distance? Sure. But how far could you possibly need to go?
I use an "ebay wireless mic", and it works fine. I also use a Wal Mart boombox on occasion. If your spectators are paying more attention to your sound quality than to you - you have much bigger problems on your hands.
Short version - You get what you pay for is usually true, but it's not absolute. There are plenty of companies that charge ridiculous prices for the same crap that everyone else sells.
If "you get what you pay for" was 100% true -- Wal-mart, Costco, Sam's Club, Dollar Tree, Honda, Toyota, eMachines, and hundreds of other companies would be out of business.