Why paid services?
There's no such thing as an altoholic who isn't a steady subscriber (money) or high-activity player (cog for LFR/LFD's wheels). Of course, the experience is usually successively less interesting each time around. How about this:
Upon reaching 90, a character receives a quest to retrieve an artifact. After a little Gao-Lai-style adventuring and a few thousand gold, a player is awarded an item — maybe a trinket — that is BoA and increases all XP by 20% up to max level. Here's the twist: every subsequent character to reach 90 receives the next quest in the series, and their awarded item stacks with the first. The eleventh character on the realm levels with an additional 200% to XP from start to finish.