Costco is often but not always the cheapest. Figure out what size, brand, and type and shop the installation places on-line. I have had Sears, Costco, the local Goodyear dealer, and a local independent all be the cheapest on the Michelins and Bridgestones we have used on the Hondas and Toyotas. Lots of places will price match or beat with a written estimate, which you can get from some tire retailers on-line.