Newegg has it for $79, a savings of nearly $20. The MS Store has it for $89, but it's taxed (at least here it is), the final cost was $96 & some change. I just installed it on my notebook today, it took only 5 minutes.
If you don't plan to move it to another computer, it's the easiest upgrade possible. The key simply unlocks what's already under the hood. You can save by buying it from Newegg or Amazon. Stay away from online auctions for this (I'm calling no names of places, just a warning).
Here's a good deal.
http://www.newegg.com/Product/Produc...-774-_-Product
Newegg, you can trust.
PS: Whatever you do, don't lose this key, as you'll need it to reapply upon a reinstall. And make sure that you either have recovery discs or a recovery partition (both would be preferable), and keep a backup made prior to the install of the key. If you have to reinstall using a different key (from the one that you upgrade), the Anytime Upgrade Key will no longer work.
Cat