It will cost more given they lack know how in house but after they hire enough people, the next cars they design shouldn't cost any more and there are advantage in manufacturing given it's easier to assemble.

Regarding VW sharing platform to spread out the initial costs of development, it sounds nice but it also dilutes their own offerings if another large manufacturer produces more desirable cars, VW will lack any brand specific feature. In the future they may also choose to just copy VW battery design and make their own for a fraction of the cost, as it's popular in regions like Asia, i.e. China so they better have a much better platform in development that won't be shared to again regain buyers. At the start they might not feel threatened because EVs are still a small majority of new cars sold but that can change very fast in terms of automotive time.