The Razer DeathAdder Essential is on-sale at Amazon in a limited-time deal for $19.99, a whopping $30 off the list price. The DeathAdder has been my gaming mouse for several years now, and tops our list of the best gaming mice, knocking the once-venerable Sidewinder off the mat.
It's lightweight, precise, has a nice feel in the hand, and features exactly the number of 'extra' buttons I want for gaming without overdoing it.
It's hard to go wrong when you're getting change from twenty dollars: the DeathAdder V2 is a more expensive option but does pretty much the same thing wirelessly (and here's a deal on that with a headset). You're getting a serious amount of mouse for the money here: it features a 6400 DPI optical sensor, 5 programmable buttons, mechanical switches and rubber side grips.