While the resin is not conductive, the carbon fibers are. (just like graphite, they are both carbon after all) Its like a big Faraday cage.

I have worked on carbon fiber bodied aircraft many times. Any time I am sitting inside one, my cell reception dies.

Carbon fiber is not that expensive when compared to milled aluminum. At least for smaller complex assemblies. Large simple parts like beams and brackets can be more expensive though. Aluminum itself may be fairly cheap, but the process to mill and machine complex parts using expensive tooling and equipment takes a lot of time and is costly. Carbon fiber may need to cure for several hours, but at least several can be cured in the same autoclave. You can only machine one piece of aluminum per machine at a time. It really depends on a good production process that can make it quickly and cheaply from the manufacture point of view. Carbon fiber is basically the same as fiber glass, but with carbon rather than silicon. It is more expensive than fiber glass of course but prices are getting lower for the components. People have been making complex shapes with fiber glass for years (think Jacuzzis and boat hulls) that means that production will not be very expensive. Bulk prices for carbon fiber will mean that the case would be no more expensive than milled aluminum blanks.