[Source] Leaked by WOW Insider last week, Blizzard Entertainment confirmed with Eurogamer that the option to purchase a level 90 character at USD $60 is indeed coming along with the undated Warlords of Draenor expansion. 1 free level 90 character will be included in the expansion.
According to Ion Hazzikostas, lead encounter designer, “Blizzard decided to sell the boost separately because it didn’t want players to have to buy two copies of the expansion in order to boost more than one character.” The “high price” was to make sure manual levelling was not “devalued”.