We were driving from Glacier to Yellowstone and didn't have a campsite.  Found this city park right off the highway going through Choteau that allowed camping in the park for just $10.  Seemed kind of sketchy when we pulled in in the dark at 10pm, but it served its purpose.  Looked like there was only one other camper when we were there.  Just needed to leave $10 in the pay box or pay online via QR code.