So you're right, apples to apples, because of the TPMS cost, it's not that good of a deal unless you keep the car for a long time. The problem with your calculations are that you assume apples to apples. Snow tires are not the same as summers. Right away that rules out apples to apples. If you have to "compromise" performance to get good snow driving tires, you're likely willing to compromise on other things for the 5 months of the year you need snow shoes.

To give you my numbers (I'll use the same mounting/balancing costs that you used)
Wheels = $300. I got cheap 17" rims. They're not super good looking, but they're effective, they use a universal bolt pattern so I don't have to remove the pin every time they swap wheels.

Initial cost with separate rims.
Wheels + mount + balancing. I originally wanted to go to tirerack for my rims and tires, but htey wouldn't sell me anything, so instead i went to a local tireshop. wheel swap costs are free. My tire place does rotations for free which includes winter/summer swaps. The place i would have used for tirerack also does the same thing, so no swap cost.
$300 + $30 + $30 = $460
Other than new tires, no yearly maintenance cost

Swapping tires on the same rims
$30+$30 twice a year. Most places I use include swapping in their mounting/balancing costs.
So basically 120 a year.

In four years it pays for itself. Beyond that. the numbers above don't include additional benefits that are not as clearly tangible. I have a G35S which has 18" rims stock. Gettng 17" rims means it's cheaper if i bend a rim on minnesota winter potholes. Also, 17" tires tend to cost less than 18" tires.