The ARRL wants to know when you do good things like provide communications for a race, Triathlon, emergency, or other public service events. There is an online web form that is easy to fill out. Just go here:
http://www.arrl.org/fsd-157-public-service-activity-report
The form will ask for the type of activity, the number of hams, and other good stuff.
There is a section on the value of the equipment used. Make a best guess. It is more important to tell them that you and your group did the job. BTW, the reason for the $ value is so that the ARRL can tell the world that this is how many dollars in equipment that we are providing to support communications. Don’t ignore it but don’t get hung up on it either.
Remember if you do something then tell someone! A little of your own “horn blowing” is a good thing in Amateur Radio. So toot away!