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My main concern is a player gaming the system by stacking large numbers of cheap cards (say, Rusty Androids).

I don't think this will be an issue. For one thing, it could provide a use and demand for those cheap cards, but also you'll be matched against opponents with similar collection scores, so if they have a more diverse set of cards then you'll probably just lose. So ideally it will be better to take whatever you might have spent on a bunch of Rusty Androids and get a bunch of more useful cards that will actually help you win battles.

The overall goal here is really to incentivize growing your collection in the intended way vs creating multiple accounts or "stacking" cheap cards. It may take a few iterations to get there, but we'll keep working towards it.

The point is that you might not be allowed to transfer all the rusty androids to other N-accounts to alter their collection score just before claiming the rewards