If you bought the identical containers with the identical contents at both stores, then yes, it would be robbery.
But, what folks generally fail to realize is additives are what cost money. 80w-90 is not the same 80w-90 everywhere. Many times the cost to manufacture a premium oil (like the John Deere) is close to the cost you can buy brand X just barely meets the specs kind of oil off the shelf.
So the question is, do you want an oil which likely barely meets the GL-5 spec or do you want a premium oil? Not saying if you pour it in your tractor, that you're going to blow it up in 10 hours, it's a question of if you want the best which will give you added protection under harsh conditions, or just a ho-hum oil.