Welcome to Just Answer and thanks for your question. The answer depends on what you buy at a warehouse club and how much each purchase saves you. As you know some things are much cheaper and others only marginally so (if at all). You would need to keep a record of what you buy at the warehouse club and what an equivalent amount would cost at your grocery store (or other retailer for things like furniture, appliances, etc.). When the amount is $60 you have hit the break-even point.
No, that wasn't what I said. Look at it this way. You want to buy a king-size mattress set. You've checked the price at several outlets in your area. The cheapest one has the set for sale at $2800. You find the identical set at Costco for $2400. You have saved $400 and have more than broken even.
On the other hand, a large package of Kleenex (36 rolls) may cost $22 at Costco, but 36 rolls bought in smaller packages (six packages with 6 rolls per package) at your grocery store on sale may add up to only $21. You haven't saved anything, but spent more at Costco than you would have for the same item at your grocery store.
There is no rule that says Costco would always be 5% cheaper than buying elsewhere. If there were, you would need to spend $1200 to break even (5% of $1200=$60). Sometimes it is cheaper and sometimes it isn't.