I can get the 5x5 BB sheets here in Colorado for $45-$50 per sheet. It's really strong, the 1/2" thick stuff is like 9-ply, so its stronger than most plywoods out there. It makes great drawers and cabinets too.
BTW Lynnrb - that mattress is awesome, I may get one....the queen is 58" wide, so would work good for the 60" plywood.[/QUOT
I used 1/2" BB on my platform. It is supported on the sides and down the center and 1x2 oak strips on the two ends. It is rock solid, no need for 3/4" BB.