Newegg has Microsoft Wireless Xbox 360 Controller for Windows (Black) for $43.99 = $7 with coupon code EMCNJHN34 = $36.99 with free shipping. 
Note, the controller itself is fully compatible with an Xbox 360, and additionally includes a USB receiver which may be connected to a Windows-based PC enabling it to use up to 4 controllers simultaneously.

Retail Price:	$ 54.99
Deal Price:	$ 36.99
Savings:	$ 18 (32.73% Discount)