Thought about building my own park once or twice myself as i know of a few spaces and i'm a wood butcher by trade.
Ged is right, the correct plywood isn't cheap and, if you are doing it properly, nor is the timber, screws and massive man hours it takes to put something worthwhile together.
If you want it to appeal to bmx peeps as well as skaters then square footage is a must. Alot of the smaller venues are too tight to be comfortable for bmx generally.
I always thought i would ban wax, food (on the park) and pissing in quite corners (bones springs to mind)
Location is key to a degree as is the balance of the park and other facilities like parking, point of sale etc
alot to think about when you get down to it and would take some investment of research time before you got to the finances