Well, the model I'm most familiar with is an insurance model. Your home insurance company has an interest in your property not being destroyed in an invasion, as well as your continued ability to pay your subscription fees. So, when most of the people in an area are insured, it's less expensive for the insurance companies to hire mercenaries to defend the area than it is for them to lose all their customers or pay to rebuild a whole city.

They don't do this right now because taxpayers are required to fund things, so they can profit from living under the national defense.