Hi all. Thanks for your answers.
My model (before being repaired by netfabb) is just prism and "spheres" (discretized) that are colliding.
@Youknowwho4eva : I don't see the link with inversion. My object is just in a thousands of little pieces ("multiple shell overlapping" just like AmLachDesigns said).
@stop4stuff & AmLachDesigns : No !
Shapeways compute the right price, according to netfabb volume, and even more (I don't know the doll / eur conversion but there is 1 euro more).
The missing parts on the image are counted in the price (otherwise it would be like a third cheaper).
Volume before repairing : 4.4 cm^3
Volume after repairing : 5.9 cm^3
That's weird because linking prisms (discretized cylinders) together should take back volume.
And netfabb repairing is destroying my object (some edges are missing) !