I didn't read the article, but how is that "cost" determined? In my car dealer example, I might spend 13% or more of net sales on purchasing wholesale vehicles from out of the local area.
If I run Quest Outdoors, I might also spend 13% or more of my net sales to to puchase at wholesale the goods I sell, and all 13% or more of that money is flowing OUT of the local economy.
My overarching point is this - unless you are a local establishment that sells ONLY a product produced locally, using raw goods produced locally, some portion of your sales will, necessarily, flow out of the local economy, if only by the cost of purchasing the goods that you then sell into the local economy, whether it be food, cars, or hiking boots.
If "cost" means a yearly or monthly fee paid to corporate, it does dilute the money that stays in the local economy, but I think the whole "buy local" can be miscontrued. If I buy something from a "chain," and by that I mean an outpost of a large corporate entity, the only local impact are the wages paid to the local employees. If I buy from the typical "franchise," the local impact is greater because ownership is probably local, even though 10% or more might flow directly back to corporate. If I buy "local," but am buying a good not PRODUCED locally, a good portion of money still flows outside the local community.