I use a Logitech headset for it (Logitech 350 I think), seems to work pretty good and can probably find a decent one on ebay for 30$ or less. It has a volume control and mute/unmute button right on the cord of the headset too so that is pretty convenient. Logitech has voip phone as well that is around 100$ I think, if you were looking for something more like an actual phone.
Theres other good brands out there too but having used Logitech products I'll just use those for examples.
I'm sure a little searching on eBay would turn up a bunch of mics/headsets/voip phones for under 100$. I'd try to stay with reputable brands though so you don't end up buying some cheaply made piece of crap.