Newegg has the Microsoft Windows 7 Home Premium SP1 64-bit OEM Software for $95 - $15 off with coupon code EMCKBJJ24 [Exp 8/17] = $80 with free shipping. OEM version is intended for new systems and cannot be transferred to another computer once it is installed.
Eighty bucks for Windows 7 is the cheapest I've seen. Think I'll snag one for a computer I'll be building soon.
It's a few pennies cheaper at Trustprice.com, and that is their regular price. Caveat: it's an OEM disk, but it works fine.
I bit when they had it last time.. you'll be locked to one MOBO. No Upgrades later on.