You do need a race licence in order to enter endurance racing. Also, many racing organizations require one expert/pro rider for every novice rider on the team, so for you guys that are novices, make sure you team with an expert/pro.
As for costs, it depends where you race and how long the race is. I did a 4 hour race earlier this year at Portland and costs were roughly as follows:
Tires: CAD$480 (US$340)
Brake Pads: $100
Entrance Fee: CAD$230 (US$160)
This does not cover travelling costs, hotel, food, nor general wear and tear on the bike (expect to do an oil change before and after the race). You will absolutely destroy a pair of tires and brake pads in a 4 hour race, so don't plan on getting any future use out of them. The above also assumes that you have all other necessary race kit (i.e. transponder, pit setup etc.).
Other tracks can be more expensive. The entrace fee for the endurance race in Seattle in September is US$500 per team.
While these costs may seem pretty steep, endurance racing is the cheapest racing on a per minute analysis (i.e. you need to run many sprint races to get the same amount of seat time you get from one endurance race).