When I set my engine to use currentEngineLayoutName = Double, it is priced correctly in the tooltip menu, but when spawned ingame as a new root part, it assumes the default cost of the part. Adding another part fixes the weirdness.

Interesting -- likely related to data caching and how stock updates the pricing/costing of parts.

Will have to take a look at this after I finish updating the plugin for KSP 1.4.