Proper blue/green enemies should give anywhere between 200 and 400 class xp I find. Anything that gives 50 or less dies too fast, you should probably find a stronger target.
Of course, there are rare exceptions when you'll get 500 on a mob, or 0 on another that normally is fine, but that's just the great random messing with miss fortune and lady luck.
You don't get very high skill increase from soloing mobs too high above your rank, I find. Best to find is the sweetspot mob that gives just the right amount of skill ups.
Rats at 1-4, Moles at 4-8, Puks at 8-10, Dodos at 12-16 is pretty much the kind of progression I try to follow.
FFXIV: Cloe Delisle
Scholar, officer of the SWAGGER Free company, Sargatanas server.