Recently I was checking for some 16" michelin x-ice snow tires for a friend, and even checking quattro and blackcircles with their 8% discounts, Costco (for this model at least) was cheaper overall.
Costco was $20 cheaper per tire out of the gate. So even with 8% off from the other various tire shops, they were still cheaper. And then the big clincher was that costco, instead of just $70 mail in rebate, they offer a $100 instant rebate. So after all the calculations, costco was cheaper.
2 years ago when I was looking for summer tires for mine and my wife's cars, I ended up getting 2 sets of Toyo Avid Ascend, and turned out that 4tires.ca was the cheapest, but a good margin (since they were offering weekend extra rebates, and for some strange reason, every weekend, many of thier tires would drop by $10 per tire back then. System glitch or intentional, I took advantage of it, and got an awesome price after all rebates for those tires.
So I tell people usually, compare, blackcircles, quattro, 4tires, and then costco. Factor in all rebates (store and manufacturer) and then decide. Instead of mail-in rebates, costco often has instant ones, which are great for those who hate mail-in. And the instant ones are sometimes a little higher than the mail-in, so they may offset the possible higher price from costco. Put everything you want in your carts, and go all the way to checkout and compare the final prices.
With work from home still being a thing, and not using my vehicle as much, I will probably hold off till next year to replace my winters. Tread is still good, but they are getting pretty old.