If it is about serialization, EmitDefaultValue = false should be enough.
For display purposes:
Create a new property called "Benefits", mark it as non-data member, implement getter.
It may have to be of a generic type, common ancestor for Car, Share, etc. As another option, you might define a new type and generate its instances on the fly when this new property is accessed.
If this approach with new property works for you, also consider implementing it in an extension class, existing in client application only, to reduce server-side clutter.