$300 - That sounds about right, if you factor in massive inflation since I bought mine - or the possibility that they have a monopoly in that area and can charge what they like....
A few years ago I paid about $80 for half a sheet (4x4), which happens to be enough for two kayaks, and then some. I think you are likely being overcharged, and should shop around some more.
Also, though I'm no chemist, I can't imagine that water, or even (gasp!) salt water, is going to have much effect on HDPE.

There are some alternatives: Some cutting boards are made of HDPE, so you could buy a few of those, if you can find some thick enough. Or, you could make your frames out of marine plywood, with HDPE connectors where the stringers attach. Tom details this on his website, and, again, just a few cutting boards would be needed.

Personally, I'd make one of Tom Yost's inflatables if I needed another stowable kayak. If I just wanted one of his folding designs, I'd make it as an all wood, non-folder. These are lighter, stiffer and cheaper to build than the folding variety, and work just as well or better. Any of his designs can be built this way.

Can't help you with the mistakes you'll make. I just make them, rant and upbraid myself for a while, then try to hide them before anyone else notices them. They never do, anyway.

Good luck!
Dave

: I wonder what the going rate is for the plastic for a Yost folding boat is
: these days. I live on the north end of the LA (Santa Clarita) I check at a
: plastic supply house and the told me it would be $300.00 for a 4 x 8
: sheet, had to buy the whole sheet. He was helpful, said he thought it
: would be about the same at a big box store. He also indicated there was a
: special formulation for exposure to water and salt water that he highly
: recommended, same price and more or less the same stuff.

: anyone got half a sheet to sell?
: Meanwhile I am slowly making progress on my S&G, would be faster if I would
: just go ahead and make the mistakes and then ask for help to fix it.
: Perfectionism really slows the build.
: thanks, John