you're making it waaaay too complicated. the standard sign is made with 4mil stuff, meaning 4 milimeters thick. there is also 2mil stuff, which is great for making wings and such. when you start talking about the thickness of the plastic and the size of the corrugations you've gone out of the realm of importance for our purposes. visit www.spadtothebone.com and you'll see MANY people using whatever's available to do crazy things. some folks use leftover signs, others purchase coroplast from wholesalers. www.harborsales.com is a dealer in coro, and they sell most popular sizes and colors at very reasonable prices.
you can make a PBF out of political signs, real estate signs, or even cigarette signs. bottom line is you can use whatever, 6mil, 4mil, or a combo of those (2mil is too flimsy) to make a pbf. the weight difference is negligable.