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Think about it some more. They aren't doing it due to expectation that the number of factory returns will be significant. They are doing it for per-unit cost reasons.
Do you think it makes sense to make them ALL convertibles?
If you can't make two options and put them correctly in different cars for less than $1500, you don't belong in the car business. You can, of course, build only one variety at a time, if you want.