If $4 is acceptable then while this isn't explicitly done, it is already available to some degree.
Currently there are
67,142 Kindle titles available at Amazon for $3.99 or less. About 7400 of them are completely free. Admittedly most of them are public domain stuff or super backlist. But you can get a paper copy however you want and bundle it with the digital copy yourself.
But yeah, that is a key option I'd like to see them offer (though each publisher would probably want imput making it difficult for Amazon). I'd also like to see the ability to transfer the "ownership" of the digital right to another Kindle user for a small fee with the split going to Amazon, the copyright holder, and the original "rights" holder. So essentially if I don't want my digital copy of Carrie any more (purchased for $6.99 I think) then you can buy it from me "used" for say $2 and we three get something, all coming out ahead with no net increase in teh number of copies in circulation but the copyright holder getting more money.