Foam pits typically get 6X6 or 8X8 foam cubes, they run nearly a buck each. A local park to me had afoam pit around 4-5 feet deep with a trampoline under the foam to absorb more shock, it was 16X16 I think, and they spent $14,000 just on the foam.
It was indoor and the foam was still SUPER GROSS after like 6 months. I learn how to rotate flips into it, and I AM SO GLAD I WENT THAT ROUTE and didn't just try to huck one somewhere. I would have landed on my neck for my first roughly 10-15 tries, then on my face for probably the next 10 tries. I had no one to teach me what to do, and just spent almost a whole session figuring them out. I got em down to landing on my bike on the wheels on the trampoline part of that pit (knocked foam out of the way) then tipped over. Never tried it for realsies though.
As to quick setup/removal, that's not really an option with a foam pit. You would want the walls to be stiff, the bottom to have some give in case you lawn dart into the foam, and to be wide enough and long enough for accidental carving/wind. Minimum of 5 feet before you hit the bottom of it. FILLED with foam.
Just build a junker bike, a kicker and go find a dock and a lake or sea. WAY cheaper. Hit the kicker and land in water.
Another option is snow if you have enough and get powdery snow. You can also look at learning into wood chips/VERY loose dirt, but I would use that as a next step after learning how to crank the rotation well.
"Hey anybody ever make that mistake like right when you wake up in the morning and you believe in yourself?" -Kyle Kinane
"BIKES!" -Tom Segura
