Suppose it depends on what car you have, where you live, what roads (surface condition) you drive most, how many km per year etc. Some of the best winter tyres you can get are made by Nokian, with tyres able to do 270kph on ice / snow... and at a fraction of the cost of some other "big" brands out there. Nangkang do a very reasonable Winter / All Season tyre with a good price / performance ratio.
Some examples:
For the Subaru Impreza I have two sets of rims, one with summers for all out summer grip and one with full winter tyres (not studded as studded not allowed in Ireland). That said if driving relaxed you wouldn't notice the difference, but on a more "spirited" run you would see significant additional grip with the summer tyres in summer conditions.
A4 Avant Quattro 3.0TDI S-Line I also had two separate asets.
Volvo V70 2.4T (front wheel drive) ran winters all year round. Although only front wheel drive, with the addition of the winter tyres it was still more than capable to go over the Wicklow Mountains military roads without issue (like Sally and Wicklow Gaps) during the winter with a few inches of snow.
A6 Allroad I ran winter tyres all year round (air suspension made it a very comfortable car but very lack luster handling so no improvement with summer tyres during the summer as could never push the car handling wise anyway.
Hyundai Tuscon Jeep (4wd) we ran winter tyres all year round... was perfect. Never too hot in Ireland to cause any significant extra wear or less grip... better in the wet then summer tyres and only thing that has sufficient grip in winter conditions like snow and road surfaces below 10 degrees Celsius.
Jaguar X-Type (front wheel drive) been running winters all year round. Current tyres have about 40k kms on them. Backs are like new and fronts are now just over half way worn so about 3.5-4mm left on them. So the wear is not excessive using winter tyres - contrary to what a load of people would have you believe.
The reasonable wear on the winter tyres during the summer is mostly due to only moderate temps and lots of rain in Ireland. Only thing is that winters make a bit more noise on the road, but with the cr@ppy "tar and chippings" roads around where I live you wouldn't notice that, only would notice it on the likes of the M50 and other motorways. You need to increase the tyre pressure by about 10-15% over what the normal tyre pressure index for that car states (e.g. if it says 2.4bar run them at 2.7 if winter tyres). If you do not do this you will have grip issues and additional wear issues (winter tyre side walls are softer due to softer tyre compound and hence need some additional pressure to perform similar to summer tyres vis-a-vis side loading during cornering etc).