Besides size, what determines the cost of creating a composite structure for a single engine piston aircraft?
Cost drivers for composite structures:


Weight: Lighter is harder to do and needs more work and more expensive materials.
Surface quality: Either you use lots of filler and several sanding and filling cycles, or you produce a high-quality negative mold, which is a lot of work.
Reproducibility: If you want to produce several identical parts, you need to produce a reusable negative mold. Simple composite structures can be built in a positive technique.


Positive building techniques

You can either use a styrofoam core which you cover with fibers and epoxy (just as Burt Rutan did), or you build a balsa wood core on a simple jig and cover it in fibers and epoxy on both sides to produce a sandwich shell. In both cases you build from the inside out and have to finish the surface with repeated cycles of filling and sanding until you arrive at the desired contour and surface quality. This is a lot of work already, but much simpler than building a negative mold.

Negative building techniques

Here you first produce a core of the desired shape (again much like the styrofoam-epoxy structure above) and use this to produce two casts which will serve as the mold in which to build the eventual structure, or you take a large CNC mill and have it produce the mold from a big block of aluminium (or another suitable material). Now you will build the structure from the outside in, so you achieve the desired surface quality without filling and sanding the final structure, and you can repeat this process to produce several identical pieces until the mold is worn out.

If you add up the material and labor cost of the two techniques, you will find large cost differences which are reflected in the quality of the final parts.