I got the Naxa single wireless microphone system at the Surplus Computers "warehouse sale" a few months ago. It was only a few dollars, and is only worth that much.
Link
JCPenny has a rebanded NX-982 for sale for $7.99 with a coupon code for free shipping.
Link to Fatwallet Thread
Two of these wireless microphones used at the same time will obviously cause problems. However, consider my mini-review for the NX-982 to get a general idea of the quality of Naxa products (which are imported from China).
I haven't figured out how to get a good strong signal out of the Naxa NX-982 into my computer, but it was both somewhat high-quality and somewhat cheap. The microphone is a metal and heavier than it looks, so not just cheap junk. The receiver module, though, feels cheap, and the 1/4" to 1/8" adapter they include is flimsy and doesn't make a solid connection with the 1/4" plug. The spec sheet says the wireless has an "Effective distance" of 15-30m. The signal was weak no matter how I setup the microphone, so I think its a matter of figuring out the best setup. I'm not sure what kind of wireless technology this uses too, but its not bad for a inexpensive, fun, audiophile toy. The jack on the microphone is XLR, and the product includes a decent length of XLR to 1/4" (stereo) cable. The receiver has a 1/4" plug also, but, as mentioned previously, there is a cheapo adapter to convert it into a 1/8" plug. The microphone and receiver each use one AA battery.
Edit: I just noticed there is a small slot in the receiver for the user to use a screwdriver to adjust the gain (i.e. strength of the output signal). Adjusting it gave me a stronger signal, but also a little more noise. This microphone was definitely not made for professionals or serious recordings. Even with the gain at the highest, I have to hold the microphone less than two inches from my mouth to get good sound. For karaoke, that's perfectly fine, so go for if this sounds like something you can use.