I don't think this is universal across airlines. For example, international upgrades on UA are NOT cheap to buy using miles.
First of all, in UA Economy, only H, M, B, Y fares are upgradeable using miles.
The ONE-WAY upgrade price for B and Y (the most expensive economy class) fares on flights from North America to Europe, South America, Asia, or Australia is 15,000 miles. For H and M fares the ONE-WAY upgrade price is 30,000 miles (also applies for Biz to First upgrade).
That same 60,000 miles on the round trip, OR LESS, will get you an Economy Saver Award to those same destinations! (Only 50,000 to Europe or South America.)
Kathy