It's still pricey. Making it hollow only helps if it's tall. For color 3D printing, the material is gypsum powder (well, there is CMYK filament now, but I haven't looked into it much). You need considerably thicker walls, etc. Furthermore, the cost is as heavily influenced by machine space as by the amount of material used.
I just found and watched your SciPy talk. The 3d-printed geology puzzle is really cool!
Yeah, the British print is basically the bottomless box you show at 15:16, but with much thinner walls - I don't have it anymore but I'd say theirs are about .3 millimeter thick. I suppose that's also why you paid three times more.
Since they don't have a bottom lid, they apply their bottom texture underneath the top.
How difficult do you think it would be to clip the print along a border, e.g. the border of Alaska?
I erred on the side of too thick walls instead of too thin, as I was a bit worried about durability. They're thicker than they need to be, though. It would definitely drop costs to make them thinner.
As far as clipping along an arbitrary border, it shouldn't be difficult. I haven't actually made a model with a non-square border yet, but I've been meaning to for awhile. I probably won't have time to in the next few months, though. If you give it a try, let me know how it goes!
For example, this was ~$100. https://raw.githubusercontent.com/joferkington/scipy2015-3d_... I tried uploading and pricing a "hollow box" version, but it saved less than $5.
However, for this one, using a "hollow box" saved ~30%, (it's a comparable size, but much taller): https://raw.githubusercontent.com/joferkington/scipy2015-3d_...