I can't find much NFL QB salary data for seasons prior to 2000.
I believe the degree to which someone is underpaid is a relative sort of thing. In other words, I would compare that person's salary to the highest salary paid to any other player at person's same position.
I'm guessing Mahomes is the top paid QB this season? For 2023, if my math is correct, Brock's salary is approximately 1.6% of Mahomes's salary.
I'd love see how much Warner's $250k salary compares to the highest paid QB salary for 1999.