Costco is one of those places where you go in intending on spending £30 and come out having spent £200.
Tell me about it. I bought an oversized hammock from there about 15 years ago at a bargain £200+
My Mrs banned me from going back

Back to tyres, I always thought Costco were the cheapest for Michelin but I went this morning to price up 4 tyres for my daughters Polo 195x55x15 and they wanted £100 a tyre, special order!!
Yet I can get them elsewhere for £87 fitted or delivered for £64 which is a considerable difference.
So not always the cheapest option.