My priority is to build a magical economy (clams, fetishes, hammers, low level gem producing globals) I use gems for other matters only once my production is huge
That's an issue that needs to be cleared. If only natural levels count then will a N4 crone buffed to N5 suffer a penalty when her Gift of Health is being dispelled?
Dispel mechanics
Whenever Dispel is cast or a global tries to replace another for whatever reason, the two spells are compared. The total number of extra gems and the levels of the casters above their spells' requirements are compared according to the following formula:
+1 for each extra gem (above the requirement for casting)
+5 for each level of skill by the casting mage in the spell's path above the casting requirement
a DRN (single open-ended die) is added to each side. The side with the higher number wins.