Any key you find with "a heavy discount " is almost certainly not legit.
It costs what it costs.
My hrrrrrnnngg levels are through the roof but I'd rather be legal
1. Any legit place that sells Windows 7 also
provides the physical DVD. If they don't, not legit. You can't buy 'just the key' anymore.
2. MS pretty much sets the price. If it is significantly cheaper, either it is 'on sale' from one of the majors or not legit.