From my understanding of it, the agency charges for 1) insurance/liability and 2) I believe the preceptor does make some of that money. Here, Sac Metro FD will take their own employees as interns at no extra cost while Sac City FD will still charge their employees. The high cost (3900$) of the "field internship" from Sac State is because of the units. Because students are getting college credit, they pay per unit. The field internship alone is 10 units.
So basically, you're looking at anywhere from 3900-6500$ just for your internship.