I can't speak to the equity differences between coaching cross country vs. other sports such as football but I can promise you the amount you get paid is much less than what you are worth coaching HS cross country. You are 'under paid' because you put in countless hours teaching, coaching and training a group of HS kids but the school's mission has to start with academics, not athletics. I am the president of our school board so I understand how public school funding works, at least in our state. If your district is like the schools in my state, it is a battle to have enough money to fund core academics let alone sports and other activities.
Despite the little amount you get paid, the value you bring to HS kids is immeasurable. My Dad was both a teacher and a coach and became a mentor to many kids. I am sure you are the single most important adult role model in many of your runners' lives.
My wife is actually a volunteer coach on our HS team. I am guessing she spends ~20 hours per week between practice, meets, writing workouts and logging data on all the kids. We both volunteer our time in the summer to work with the Middle School and HS running program as well. We do this because of our knowledge of the sport and passion for the kids.
I'm not suggesting any HS CC coach should volunteer and do this for nothing, but I am saying you are never going to get paid what you are worth so you should be doing this for other reasons.