If you have a genuine lumber yard slice a 4 x 8 sheet of shop grade Birch/Maple plywood into 3 1/2" strips 8' long it will yield 13 pieces. The per piece will come in about $4.50 to $5.00 aprox. This will provide the most stable of benchwork frames.
Solid dimensional lumber will warp. Can not give an account of the many solid wood layouts I have had to straighten.
For decking the best deal is Advantech at 84 lumber for about $30.00 per 4 x 8 x 3/4" sheet. The density makes it much quitter than 1/2" plywood or OSB.
For demonstration purpose I built a 200' long around the wall layout using all popular benchwork methods and you can hear the difference as the train moves from one construction style to another.
1 x 4 (3/4 x 3 1/2) plywood sides and cross members on 16" centers
13' x 29' staging loop. Floor taped to determine aisle size restrictions.
Predrill wire management holes and run most wires b/4 decking.
Below: beads of silicone on all mating surfaces with decking pre positioned on dowels and gently clamped in place with gussets holding plywood butting edges even. Helps in sound control.
I have now retired from benchwork to finish my own RR.