Dell.com had the PS2 version of the Logitech USB Mic for $15, I picked it up and it works great. Unfortunately I can't find it on the site anymore, so it may no longer be available. The PS2 Logitech Mic works, though, you can be sure of that.
I am pretty sure the Karaoke Revolution one works for it. If you like the singing on rock band I would suggest picking up the new KR for 360 w/mic for 60 at target. Good deal mic for rock band and another great singing game