I just insured my '02 700 Edge X through Nationwide - full coverage and liability for me was $430/yr with a $500 deductible. It would have been $100 less, but I have an 83 in a 55 mph speeding ticket (automotive) on my record, and the assumed 'recklessness' of on-road carries over to the snow...
Even if you think you're a safe/careful rider, I would recommend insurance. Aside from the possibility of theft or somebody clipping your sled, the proof for me was something I saw firsthand last year. We were at a sled dealer in Houghton MI installing a shield heater, when we heard a sled take off WOT in the parkinglot - looked up and nobody was on it! The sled was a rental, and they pulled it out of the shop for a new renter. A couple pokes of the throttle before starting, and it started fine, but moisture on the carbs froze them open. It did an empty drag race about 150 yards into a parked Ford Taurus (empty), and knocked the car sideways, bucked in the front drive wheel, and mangled the quarter panel. Sled was an explosion of aluminum and plastic.
So 2 lessons - 1) get insurance, 'cause stuff happens, and 2)
ALWAYS attach your kill switch tether to you before you start your sled!