The Outpost

I haven't worked it out entirely, but there's three main things that limit my economy from busting:

• Most magic originates from spirits, who are people in their own right and expect pay for their work because magic is exhausting.
  There are other ways to obtain them but unless you're making magic items then they won't be any better than spirits.
  
• Magic can't do everything, only a set amount of things based on the color of the spirit.
  
• Only certain colors of spirits are in high supply in certain areas, so tasks not suited to their color has to be done manually.
  

So spirits serve the jobs they can do as specialists and other positions they can't fill are done by regular people.