Marginal costs involve market goods, of which a university course, is not.

Unless you can show that the good (course) would not be offered if the foreign students were not present, there is no implied cost/value proposition to build a break-even..

In addition, for many courses, there are scaled costs that are not pure margin contributors. Most medical, science, and engineering courses have costs which rise with each additional student.

Yes, the professors' salary is fixed, along with a few other factors, but the calculated costs which are used to develop the "90%" subsidized are so large relative to the average course size (psch 101 with its 145 enrollees not withstanding) each course would have to be much larger than they currently are for the marginal contribution to be relevant.

The only "market aspect" of the cost of a Canadian course for non-residents is a perceived value relative to universities elsewhere.

Short Answer - Trent can't cost more than Harvard.