Well considering a legit box from fr*sh tip is like 2 g's.
PVC piping is appx. $1 a foot
Shitty 2x4's under 2$
Trees - Free ex. hitching post / pole jam.
I would try to not damage city property ex. picnic tables / bike racks for the sake of longevity of your set ups. Maybe just have them around to set up on the fly.
Find someone who welds, you can always find piping.
If you want to be super legit-contact the city about it.
Duluth just got a public cement skatepark because a couple of kids were super-pro-active for a couple of years and got what they wanted. I'm sure if you were determined enough you / the ski community could turn out something great if everyone pitches in.