Saying it once, and saying it plainly

Give the figure as a fact, not as a negotiation or a test. "We can put this much toward your education in total, across all the years. Anything above it has to come from a scholarship, from your own earnings, or from a course that costs less." Then stop talking, and let them work with it.

Two things make this land badly and are worth avoiding. Attaching conditions about which subject they choose turns money into leverage over a decision that should be theirs. And repeating the number every few weeks turns a boundary into pressure. Say it once, clearly, and then be a resource rather than a reminder.