What?
How is buying a second set of FOUR tyres costing £100+ each for one week of the year not a waste of money? It's exactly twice as much more expensive (probably more) than owning one set of tyres.
That's like buying professional diving equipment for the one week of the year that you might possibly spend diving in the Bahamas.
It's a waste of money for everyone who doesn't have an above average disposable income i.e. the vast majority.
Two sets of tyres last twice as long as one set though. There are extra costs, but they are minimal:
Based on 10k/pa in an E46 in my case these are my extra costs:
Storage - this is free for me
Extra fuel - this is MINIMAL, I never noticed it at all in my audi, but potentially ~1% perhaps an extra £10 pa (only half your mileage pa is on the winter tyres remember)
Rim depreciation - I bought some second hand BMW rims on ebay for £100. If I keep the car another 3 years, I expect to sell those rims for maybe £50. Extra £16.66 pa
Tyres cost ~30% more per set - assuming I can make two sets of tyres last 3 years, that's pretty easy. Summers: £80x4=300, winters: £100x4=400. so £33.44 pa extra over just using two sets of summers.
So in total, I think having winter tyres will cost me about £60 per year extra. To be honest - I think that is worth it for the extra grip you get when its cold/wet and the fact you know if it snows you don't need to worry at all. That's for premium tyres, V rated, 205/55/16. If you have a Golf or something on 195/65/R15 and you only want H rated, your annual cost will probably be about half what mine is.