Empowerment is extremely expensive.
Empowering from Level a to Level b costs 15*b gems if I remember correctly. So going from nature 2 to nature 3 costs 45 gems nature 5 to 6 costs 90 gems.
A thistle mace which gives nature +1 costs 10 gems to make.
Might be of course that you can't use the booster cause your mage doesn't have hands (treelord for example) or you have to little magic to forge the booster from the start.
edit: dang, to slow
