Did an instance, nothing gave experience. (Level 50)

[Snugglebutt]

You DO however gain exp from the level 15-49 dungeons, both from the mobs inside the dungeon and from the lowbie roulette. I dinged 51 in baby Haukke Manor.

It took around over 2 million EXP to go from around 55-56 and almost 3 million to go from 56-57 (I'm only 57 at the moment) so the novelty of using lowbie dungeons would wear off pretty quick trying to level to 60 that way.