Materials
Steel
At http://www.metalsdepot.com the cost of shipping doesn't change much based on quantity, at 6', it's $32.44 at 240', it's $35.65.
A 12' section of 3/4 steel tubing is $15.64. It would weigh about 8lbs, a 12' 2x4 would weight 26lbs and cost about $5.
I can get 3/4" round steel tubing on EB for $1/ft, including shipping.
Aluminum
I can get scrap aluminum on eBay for about $2.20/lb and can be worked in a Foundry.
Plywood
- 1/2"
- home depot 4'x8' ~$31 or $1/sqft
OSB
- 1/2"
- home depot 4'x8' ~$16 or $0.50/sqft
Pine
- 1" x 10" x 10' $15 or $1.80/sqft
- 1" x 8" x 8' $7 or $1.30/sqft
Construction Yellow Pine
- 2" x 12" x 8' $11 or $1.38/sqft (at 1.5" thick)
This is actually cheaper at 8' than 12' and the 12" is cheaper per in. than the 10", plus those extra 2" are interior which means more usable board.
Douglas Fir
- 2" x 10" x 8' $8.22 $1.23/sqft (at 1.5" thick, plus a little more for the edges)