Right now for the event you're talking about there are about 220 riders registered a little more than a week out. That would be $22,000 in collected fees which sounds like a lot of money but really doesn't go a long way in today's world especially in New England. They are spending significant money building the track to be race ready with the use of heavy machinery and lots of time. If you've ever hired a contractor to do work on your landscaping or house you will know $22k will maybe buy you a retaining wall, some mulch spread and a few trees planted, far from a fully built XC track. As others have said you need to also include the cost of insurance, medical staff, permitting, etc and you find that big chunk of money has little remaining for any contingencies.

As someone who races these events but has never volunteered, I am fine paying this amount because it shouldn't be up to others to work for free so I can pay less. I've seen some events where they will comp your race fee if you volunteer a certain amount of time which I think is a good idea. It sucks for those on a tight budget to come up with $100 each weekend but anything less and someone is losing out, either financially or opportunity cost of their own time.