Nestle race in Springville ($8)
http://www.artcitydays.com/Info.htm
Freedom Run in Provo is only $22 for a 10K, plus you get an obscene amount of freebies and goodies
http://www.freedomfestival.org/events/freedom-run/registration/
The Hale Center Theater Runs are $35 for a 10K + you get four tickets to their shows, which basically makes the race free.
Sure, a lot of 5K's are $20 each. That's about the cost of one gym membership for one month. The money is usually also for charity or some kind of fundraiser. If you don't get a shirt, it's often cheaper.
Nothing's free, but running is by far the cheapest exercise.