Though not suitable for Classroom use, Logitech headsets are an inexpensive, easy way to get quality audio at your desktop. There is a range of models available. Some to consider:
ClearChat Comfort USB - $40
Not their lowest cost model, but the lowest to include a Noise-Cancelling microphone, which is recommended. A solid basic model with no bells and whistles.
Premium Notebook Headset - $60
Featuring a slightly lower profile than the ClearChat, the two selling point of the Premium Notebook are its portability (it even comes with its own case) for times when mobility is important, and its connectivity options. The Premium can be connected through an analog microphone jack, or through USB using the included adapter.
ClearChat PC Wireless - $100
Don't let the name throw you, the ClearChat Wireless works on both PCs and Macs. This is Logitech's premiere headset and combines all the features of other models with 2.4Ghz wireless.