I'm not sure your proof works. In the absence of collusion, you'd expect vendors selling the same thing to ultimately end up with the exact same prices. After all, why would the customer keep buying from Player B if Player A is selling the exact same thing for less? The only way Player B has any chance of finding any customers and dividing the market, without some other differentiating factor, is to sell at the same price as Player A.

If anything, Costco's higher prices is more likely to suggest collusion (but still does not imply it).