KSA--you are right, that buying miles this way would be cheaper than earning them by traveling, but for the flight, you are getting the flight PLUS the FF miles. therefore, you are not just buying the miles from the airline. If you look at the value of the miles themselves, FFers may try to use a 25,000 mile award for a ticket which might otherwise cost them between $400 or $500, somwhere between .015 and .02 in value per mile in the award.
Therefore, in your example, if your flight is costing you .08 per mile, the mileage you are earning is worth about .02 per mile, so you are paying the remainder for the flight you are actually on (about .06 per mile).
However, you can't compare the two, because one is the cost of flown miles (actual air miles) whereas the other is the worth of FF miles (where 25,000 "miles" may get you a flight that actually goes only 5000 real miles round trip).
Looked at from an earning perspective, .02 is a generally neutral figure, neither good nor bad, for purchasing miles. It's just about what they are worth to you.

That's my two cents worth.

Djlawman