I'm not a CostCo member, don't have one here in our neck of the woods but my brother in law is and on a recent trip I went to CostCo with him and noticed they carried a safe with Winchester on the front.
Hard to evaluate because the safe was locked shut and nobody knew how to open it so I couldn't look at anything except the pretty paint work. Don't know how much volume is inside, what the fire rating is, what the rating is on the lock mechanism, etc.
For that kind of money, I want to know a bit more about it. Doubtful you will get much knowledgeable help from the sales staff at CostCo so do your due diligence and Homework, the price doesn't seem bad but if it doesn't have the fire rating then you might be just better off with a steel cabinet. The reason to have a safe over a steel cabinet bolted to the concrete is the fire rating.
Remember also you are going to have to deliver it to yourself, make sure you can handle the weight and getting it located in the house.
Dan R