I found this handy doc to track exactly how much "real" xp you get from which floors based on your "real" level.
https://docs.google.com/spreadsheets...gid=2137694813
So basically, if you're level 60 for realsies, and you do floors 1-10, you'll come out getting 324,000 xp for real. On the other hand if you do floor 51-60, you'll get 583,200 xp. Almost twice as much. But the real selling point of the floors 51-60 are the extremely quick queue times so you can chain run them very fast for xp.
That doc is worthwhile to have handy though because if there's a class you don't really like (or aren't particularly good at) but want to level it just to have it leveled, you may opt to do lowbie floors where you'll have only the most basic of abilities to worry about.