Seems that hempcrete has an R value of around 2.5 per inch, about half as good as XPS foam board. I'm not sure what 2" blocks cost, but eBay has 3.5"x7"x14" for about $10 (plus shipping)
, which would provide about the same insulation as 2" of XPS. It would take 47 of these blocks to cover the same area as a 4'x8' sheet of foam board - so it's $470 (plus shipping, plus losing 1.5" more headroom) for hempcrete versus $30 for XPS board. You might need 20-ish 4'x8' sheets to do a bus, so that's $9400 to insulate with hempcrete.
This seems in the same category of ludicrously expensive as aerogel, except aerogel is actually twice as effective as XPS instead of half as effective. Maybe I'm missing something and there's a much cheaper source available somewhere, but this stuff would have to be fifty cents a block to beat XPS on price.
Perhaps doing your own bulk pour is much cheaper? But apparently you can't use it structurally then, since it takes a really long time to cure. Then you'd need to fill in between studs on the floor, might as well use fiberglass then.