What about those online 3D printing services (like the one thats selling the 3D printed raspberry PI cases) - i thought those were reasonably cost effective as far as ad-hoc goes?
Shapeways is $1.40 per cm3 for plastic. You only pay for the actual material. Pi cases are easily done as even a cheap ass (£1200) printer can manage the size required. A far more expensive printer is required for larger cases. Even so, I find Digiman's prices to be well over the top. I wonder if these prices are for a solid lump rather than a hollow case.
...mmm... OK, I'll take that back, I just did some maths and I don't think he's far off after all. Ouch!
