$20k is too much. Fortunately the government agrees. At least on the Parent Plus line, you can only borrow $65k for the degree. This is new this year with the Big Beautiful Bill and I think good - but still too high. So many can’t escape the weight of such high debt. There’s the loans the student can get too.
Loans have fees - so you don’t get what you are borrowing. And interest - and you’ll have grad school -which may or may not be funded.
$80k, which means you’re borrowing more - at 8% you’re near $1k a month for ten years.
In today’s world, with grads barely surviving without loans, it’s too much.
And you’ll find schools that cost less.
In the non math classes, what kind of rigor does your son have ?
Brandeis, statistically, will be a reach but maybe a reasonable one.