You're unlikely to find a cheap mail-order source, so you would need to call around locally. However IME there is very little price difference.
Don't lump pvc and polycarbonate together, they're quite different. PVC sucks for roofing, especially anywhere in the North and anywhere that gets hailstorms and if you want it to last more than a few years. (That includes pretty much all real-world situations
) Whereas polycarbonate, the more expensive stuff, is quite durable if installed right.
You might want to consider metal instead. Plain galvanized metal roofing panels, especially if you scrimp on gauge if you are offered a choice, are typically your best compromise of price and durability (they are in-between the price of pvc and the price of polycarbonate, usually, and at least as durable as polycarbonate).
Whatever you end up using, it is ABSOLUTELY ESSENTIAL you find the manufacturer's specs for how to support and install it. I see soooo many BYCers with very incorrectly installed roofing, much of which blows off in the first good storm. Don't do that. Do it right, make it last
Good luck, have fun,
Pat