If you were to buy one that's, say, a year or so into it's life at LensRentals, it's likely a lot less.
They explain in the blog post that you can see an expected price range right on the website before rental. So for instance, on the 5dIII page, you can click "Get Estimate" to see that the expected range is $3,319 - $2,513 depending on the age of the particular copy you get.
I would expect them to price these a little higher than a straight linear interpolation between purchase price and Lens Authority price, because they're losing out on potential rental revenue for the rest of its lifetime, and they need to recoup their costs to procure a new lens and process, test, and tag it. But I still think for many it'll be worth the extra money to get a lens that's been tested, possibly optically adjusted, and that they already know they're happy with.