Sorry, meant thickness, not density, albeit it holds for both. Double thickness = roughly 2x price.
Confirmed by CAD's post, stuff's getting pricey.
Still think it's not a bad idea to use 100mm Uthermo 6 or 3 for super chunks, never seen anyone do it though, not sure which one would work best.
Timber_MG, lower density stuff seems like your area, could you shed some light on the merits?