A bunch of us just put on studded tires in late Fall and leave them on until early Spring. Figure at least $100 for a pair of studded tires, and don't expect to swap tires frequently -- handling studded tires is literally painful.

They're slow, they're heavy, they're noisy, you have to treat them more gently (I've lost a couple studs riding to lateral stresses as from riding off/down a sloped sidewalk shoulder to the roadway). I find myself riding slower in part so I'm less likely to need a panic stop (which I trust, but on clean pavement that could mean losing a stud or two). But it's nice not having to pay any attention to the weather, knowing that the studs are there if the roadways get bad overnight or while I'm working.

-Peter