Actually, I used treated fence boards from Lowes (6' dog-eared ones in huge piles in the outdoor lumber area). They only cost $1.65 per board. So if you do the quick math, you're getting 3 square feet of coverage for about 55 cents per square foot- and a great look in the process, plus it is treated lumber for long outdoor life. A sheet of 4x8 subfloor plywood costs around $20 which is around 65 cents per square foot and doesn't look anywhere near as nice. The big difference is the labor of course- you can slap a sheet of plywood up and you're done... doing a bunch of lap siding takes a lot of time and cuts.