I've been having a little look, with some help - at printing costs for physical books. I think the best way to handle this would be by means of an 'add on' amount rather than pledge tiers, as many people have selected various different levels and I'd hate for it to become confusing.
So, much like the Elite campaign - to order a physical add on - use 'manage my pledge' and add the relevant amount to your pledge without altering the reward level. When the funding ends, I'll be contacting people to find out what they want to do with any extra pledge money I don't already know.
Having looked at some printing websites that offer short run book production (and of course Royal Mail!), I think the following prices should cover the costs of doing it:
+£12 = Paperback & Delivery (Signed for) to the UK
+£14 = Paperback & Delivery (Standard Airmail) to EU, Eastern Europe & Russia
+£18 = Paperback & Delivery (Standard Airmail) to the USA & rest of the world.
I'll look into the cost and possibility of producing a hardback, or even book posters - although we might need to do these things post Kickstarter.