If we're counting 10 minutes as the crow flies, then in the east cupertino/west SJ area, you're probably in range of 8 costcos, including a business center one.
Granted, with bay area traffic, that's more like an hour of driving, but it's still pretty dense.
But now that I think of it, you can probably get that in Southern California in multiple areas. That's a circle with 314 sq miles inside.