I'd actually go the opposite route level-wise. Too ez to get to lvl 90, why pay for it? The grind really starts after that point. Move the level cap to 97-98. Experience-wise make it provide like 1/5th - 1/10th of required experience to next level, not just one set amount or next level. Or possibly make it work like an Experience Shrine on steroids for a set amount of time, give it an aura (not shared with party, but possibly with hireling (or not if that complicates things)). Make the time stackable with multiple buys but not the experience gained.