I'll leave the math to others and speak to the Winter. I live in Denver and I'll second that you do not need snow tires. You need a decent tire and as others have said, learn how to drive in the snow. I'm a proponent of Denver's public transportation. RTD regional routes for buses are excellent, there are many express buses at peak times, and light rail is expanding and getting better every day. Visit RTD (link below) and see what you think. If you use this daily, there's a monthly cost to factor so compare with the ability drive.
We've had a very mild winter this year, that usually means a not so nice winter is around the corner. It can get very cold here, dangerously cold. It's better to buy good gear the first time, things that will last - gloves, beanies, jackets and one key to moving here is layers. Just as many people are moving here, many are leaving. Check craigslist and other second hand sources, especially for the kids gear.
As long as you live in an apartment, there's no snow shoveling in your future, but depending on your parking, you might need something very basic to shovel snow to get the car out. You'll also need a decent ice scraper for the windshield and other windows. Keep in the car October - May, regardless of the forecast.
www.rtd-denver.com