There are plenty of cheap and free underground events. The easiest way to find them? Start making friends, and do the following:
Become a Regular
Go to your local bike shop and pick the staff's brains about races, events, and gatherings. (Bringing a six-pack along from time to time never hurts.) And don't forget to check the shop's bulletin board. There you'll find all manner of grassroots races and, more important, in-the-know riding partners.
Join the Club
Most local riding groups are open to everyone, and while the clubs often participate in official paid events, some members like to compete in underground ones as well. American Trail Running Association and USA Cycling are great resources for teams and clubs nationwide.
Get Your Hands Dirty
The people taking care of trails are knowledgeable about the local race scene, too. The American Hiking Society focuses on the preservation and creation of hiking and running trails. Cyclists: check out the International Mountain Bicycling Association, which keeps a list of trail-building events on its site, and advocacy group the League of American Bicyclists, which has member clubs across the country.
Sniff Around Online
Mtbr.com, dedicated to all things mountain bike, posts detailed regional listings of trails, while Bikepacking.net keeps track of endurance routes around the U.S. At irunfar.com, you'll find information about local running clubs. To find truly underground events, join these sites' forums. No matter how secret organizers try to be, eventually someone will let it slip in a chat room.
Start with Small Races
Sure, the hush-hush races are the coolest, but there are plenty of gateway events. The Southwest Endurance Series is a loose-knit association of locally run, low-key bike races across the western U.S. The movement is spreading, including a newly formed Virginia Endurance Series. Ride a couple of them, hang around for the -parties after-ward, and you'll be on your way to the lesser-known invitationals.