I did this in another thread. Check out the following math:
Without heirlooms but having guild xp boost, what's the average time to go 1-90? No, don't get all epeen about this, what's the AVERAGE time for the TYPICAL player? Let's leave out professions, as we'll define WoW's Main Story as getting your character maxed. The answer (using threads like
http://eu.battle.net/wow/en/forum/topic/5493111009 as data) is about 4-8 days (100-200 hours) play-time. Let's average to 150 hours.
How many months is that for a typical player? Let's round to two. Two months to go 1-90 seems right, especially for an average player's first or second toon. Two months is $30 of subscription.
They are charging $60 for the instant 90, but saving you the time it takes to level. Is the extra $30 worth having it instantly?
I think they overshot it a little. Perhaps it's because this gives you a max level toon right now. In WoD, they will likely lower that price, since you will still have to level the other 10 levels. Even if they don't lower it, I bet they will bundle the character boost with the expansion and only add $30.