Evaluate your family financial situation and figure out a price limit on college costs without compromising your retirement or other financial needs. Be sure to consider multiple kids’ college costs if you have multiple kids.
Run some net price calculators on various types of schools (e.g. your state flagship, local state university, some realistic-for-admissions schools other than in-state public universities) to get an idea of what kind of costs after financial aid you may be looking at.
Once you have your financial limitations, communicate them honestly to the student before s/he starts making his/her application list, and make it clear that admission but too expensive is the same as rejection.