For $60, level 90 boosts definitely seem pricey -- costing more than the expansion itself -- but DisRuptive1 on Reddit has taken the time to break down the math on that $60. To do so, he's taken careful measure of the time it's taken him to get a new character to level 90, plus grinding up professions. And though your leveling speed may be better -- or worse -- than his, if you count up the hours of playtime it takes, you can see how a boosted 90 might be worth the pricetag... at least if you really want another alt.
The short of it is that boosting from level 1 to level 90 will save you about 84 hours of playtime, while boosting from 60 to 90 and getting max-level professions will save you about 70 hours of playtime. If we take to the goblin philosophy that time is money, than that many hours certainly add up to a chunk of change -- though we'll leave you to do the math on what your own time is worth. So if you've spent enough time with leveling content that none of it is terribly new to you, then maybe spending $60 instead of spending 70 hours time isn't such a bad idea.
For all of the nitty gritty details, head over to read DisRuptive1's full post on Reddit.