With my KS for Psi-punk: World's Edge Arena I offered at-cost coupons to get the POD for anyone who got the PDF. I already price my POD books at very little above what it costs to print them (plus the cost of the PDF), so backers are getting it at a slight discount but have to work a little more to get it (as opposed to the alternative of me printing and shipping it to them).
As a layout person, I can indeed attest to teh fact that it doesn't cost much extra in terms of time or money to ready the PDF for print as long as your original PDF files aren't much different than your print files. Here are some key differences:
* The interior file needs to be stripped of all interactive elements.
* A separate cover file needs to be generated. You don't print a front cover and back cover, but instead prep one file that contains the back cover, spine, and front cover all on one spread. If you're not doing layout yourself and you're paying a professional to do this for you, that could incur extra cost. It's honestly not a huge amount of work though so it shouldn't cost a layout professional too much extra to do this for you, and if it costs more than like $50 then IMO you're getting ripped off.
* You have to order one, sometimes multiple, proof copies. That means you spend a few dollars (and several weeks of time) to print an at-cost sample of the work and ship it to yourself. Depending on the book in question this can be about $8 - $16 USD give or take. IMO, that's not enough of a cost to justify charging more or even asking for more in the KS.
So why do people charge more for the POD and then only give you an at-cost code?
As a publisher and layout professional, it beats the heck outta me.