Shure has a new wireless UHF system that's only about $400. It's a comfortable headset, and has the same freq specs as an SM58.
I tried using headsets several times, and I just can't do it.
The sacrifice of not being able to cough, sniffle or converse with a client momentarily was too great. IO also had a very difficult time getting the proper dynamics needed for my limited range. I need to work in and away from the mic to compensate for volume changes, and that's impossible with a headset. If you are the type of singer that can "cruise along" at a constant level ..... more power too ya, but I need to whisper, and scream, talk and croon .... all in the same show. A headset cannot give me that flexibility.