its 1k+ per mob if rested only, really doubt they nerfed it to half, but i could be wrong.
When fully rested, it took 3 runs to use up all rested xp, that was with my booster+2 toons/slaves + 2 friends being boosted from level 59-62 roughly


I did a quick test in stockades boosting 2 randoms, result where roughly this,..

lvl 85 mage boosting: 85 xp/mob
lvl 70 mage boosting: 90xp/mob
lvl 60 mage boosting: 100xp/mob

Lvl 85 340 ivl geared specced, gemmed etc, 1 shots everything.
lvl 70 mage needed 2-3 hits on some mobs (this was spending random talents in frost and spamming arcane explosion/cone of gold.
lvl 59 mage, well lets just say badly geared is an understatement

TL;DR
you can get 20-30% extra xp if you happen to a decently geared level 60 or 70 char to boost the low level stuff with, haven't tested yet in the higher level instances.