Pretty affordable USB Mics
Blue Yeti Nano (Shadow Grey)
Some people prefer the Rode Mini over the Yeti Nano for singing and streaming. Here's a sound comparison.
An XLR setup will be better but that's pretty intimidating for most people.
Check out gillyt's post on Blue Yetis if you're looking to pay a bit more for functionality.
Original Coupon Deal
The Razer one was $80 a few days ago on Amazon.
That better value.