My medium size town has a certified mile road race one a year and charges $10. Early registration is $8 online, but the online registration fee is like $4.00. So it is cheaper to actually sign up day of. You get a crappy T-shirt that I give straight to the salvation army thrift store. But I LOVE the event since I was a 800/miler in college and don't really like 5ks and up very much.
Another good idea I've seen is having the prizes be free entry into the next years race or another race your running club puts on.