discussions about the prices sound like:

All cars should cost the same. No matter if its a Ferrari, a Toyota or a Dacia.

Because the Mercedes (Starcraft) does not include the
Leather seats, Entertainment center and navigation yet, its overpriced.

It should cost no more than new Dacia with the FM Radio and
the Papermap.

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Its simply a different real value in gaming terms.
Wining about the missing campaigns is not appropriate.
Also as the Addons will not just hack together some missions,
but put a lot of other features and improvements into it.