You already get one level 90 boost if you buy the Warlords of Draenor expansion, but in order to allow players to level more than one character without buying multiple copies of the expansion, Blizzard is selling the boost separately at the same $60 price tag. Why $60? According to designer Ion Hazzikostas:
If our goal here was to sell as many boosts as possible, we could halve the price or more than that - make it $10 or something. And then hardly anyone would ever level a character again.
But levelling is something that takes dozens if not over 100 hours in many cases and people have put serious time and effort into that, and we don't want to diminish that.