I think the idea of tying the amount of income received through performing
common magic tasks to civilization level sounds reasonable. I would further
suggest that the available income be subject to a pool effect -- much like
recruiting is now. In other words, the population of a province/city has
a certain amount of available cash to spend on common magic services. Each
mage which offers their service must compete with all other mages offering
such services. They have to split the income between them.