I just bought a copy of Win8 Pro from Dell, via my wife's work discount. We paid $70. It was claimed to be marked down from some ridiculous, sky-high price of $200 or $300 or some such nonsense.
Cheap options include asking a college student to buy a college copy for you, or look for discounts through your employer, or your friend's employer, etc.
There may be cheaper ways, but these are the methods I'm aware of.