The 18" low profile tires usually run $25-$35/ea for mounting and balancing, so that's like $125 per swap. The ATS comes with free tire rotations every so many miles, which is them swapping wheels (with tires mounted). Your dealer may be willing to "rotate" them by swapping the winter and summer sets and billing it to Cadillac corporate as your included rotation interval. That's up to your service department.
So two options:
(A)You swap every fall/spring, at roughly $125 a shot. In my case, that's Nov'12, Apr/Nov'13, Apr/Nov'14, Apr'15 so far. So I'd have spent $750 in swapping my tires over by this point, and both sets of tires have some life left in them (though the snows will last longer than the high performance summer tires the car came with). I'd guess they'll last me easily another year, so lets say $1000 in swapping them.
(B)I bought a set of acceptable alloys from tire rack on clearance from a decent company for $120 each. They mount/balance for free with my winter tires. So I spent $480, and if I beat the snot out of those wheels with curbs in winter, I don't care. My dealer does one swap for free a year and I pay for the other one at $25. So by this point I've spent a LOT less than having them swapped, and have done less wear/tear on the rim of the wheel - by the end lets say I'll have spent $600. It's $400 saved and less worry about curbing a rim. The longer you go this route, the more you save.
Also, FWIW some dealers will happily store wheels/tires for you for a relatively small fee or even free.