You need to think about what you're giving for an award.
I'd say the worth of an object should be related strongly to how hard it is to get the object - the more of a fight/challenge it is to obtain, the more powerful it is. Therefore, if it's an awesome artifact of doom, enemies should provide insane challenge.
Of course, enemy toughness is tricky. A base that I do is take the enemy, plug in a level, and auto-balance. Then I uncheck auto-balance and tweak the stats to the enemy. I usually lower the health and raise the amount of damage it can deal in a single hit (and raise the fatigue), because I prefer my enemies to have more oomph but not last forever.