Mostly due to getModuleCost being called before module is initialized and thus it has no data to supply to the method.

Might just check if the module mass/cost fields are uninitialized and return zero to allow use of the part.cfg specified cost value.

Many of the same problems likely exist with the mass specification for these parts; should likely use a reasonable value for the part that represents the default prefab part (not including tech-limitations) so that users know at least a bit about how much it costs/weighs from the tooltip alone.

Fixed for engines, tanks, srbs