a quest like the Cactus Apple, which you can get at level 1, give you 380x3=1140xp, for picking up 10 apples. (about 1 min total pick up time if you're killing mobs there anyway).
Note that this is a level ONE quest, and it would give you about 15% of your level TEN needed xp. Imagine what a level 10 quest gives.
Also note that there are a crapton of killing quests that do the same or better, without costing any time at all, since you're killing stuff anyway.

on top of that, quests give you rep with horde factions, which eventually will lead to lower repair, training and item costs, by 10%-20%