The first microphones I bought from Jon a Naiant were around $25ish? for the pair. They were electrets built into XLR housings. Great mics, I actually used them over the weekend. Karma used to have some really cheap mics too.
If you are considering those mini-jack mics you could also try plugging a really cheap pair of headphones into the mic jack. They will transmit sound, like a mic does. But, you will probably fry the headphones if the mic jack outputs DC for electret mics.
Anyway, good luck and keep looking. I bet you'll find something that'll work for you. Check eBay for used mics too.