There are no cheap MS licenses. Whatever cheapy you get may work for a while, then at some point, MS catches up and then you stop.
I buy OEM from Amazon, look at the reviews, it should say it will install on a new machine. OEM says you get no support (who calls MS for support these days, be on hold for 45 mins, and only get the "company's line" anyways), and it suppose to work for your installed machine only and not transferrable, even if that box dies, but in reality I found it can be transferred, albeit needing a longish re-registration process, needless to say, one active box at a time.
You get a "student rate" from these OEM, 30-40% off retail. Don't jump on it right away, the prices go up and down.