You would need 4 4000 gal trucks, up here in Canada they would set you back about 140 per truckload - so close to 600.00.
For plastic barrier, you can go relatively thin, i used the typical 6mil house vapor barrier.
I like using screenings or crusher run for the base plates to sit on - packs very solid with a plate compactor and some water, but as Dennis said, your existing top soil will work as well.
Your metal straps won't straighten by setting weight on them, but try hammering the crease flat on your garage floor or something similar,(hammer from both sides alternating) that's one way to get creases out of flat metal without the proper equipment to do so.