I recommend USB headsets over the other kind because they tend to be less quirky. They’re plug and play ready and that’s always handy.
I’ve been using this one for the last year or so, it is only $30: Logitech USB Headset. Sadly, I see the price on this one has gone up so they must have discontinued this model.
There’s a Plantronics that looks affordable. I like these behind the head models, they don’t mess with your hair ;)
If wire free sounds good to you, we’re hearing good things about this one: Plantronics Audio 995 Wireless Stereo Headset.