I cant fathom why you would want to keep spending $15 to have a level 90.. the level process is seriously not that hard..
I would never do it, blizzard gets enough of my money as is
Depends entirely how you value your time. If you get paid $7.45/hr and the cost of WoW is BattleChest (20), MoP (30), and WoD (40) we can determine it would be $90 to get a free level 90, meaning you would have to work 12.1 hours to break even. It takes AT LEAST 15 hours gametime just to ding 90 with heirlooms and Darkmoon Faire buff.
It'd probably be worth it for most people, especially since "powerleveling" costs hundreds.