**Update** Much cheaper than the following and in response to this and a couple of other Stateside comments, your best bet from America is to go here.
The third-most expensive way to buy KSC is as an eBook: at £9.19 it is a mere 20p dearer than the full-price paper version.
The second-most expensive way to buy KSC is to go via the Sunday Times website, where they seem to think that if you'll pay to go through the paywall, you'll pay anything, and the price is £11.69.*
The most expensive way to buy KSC is to get drunk sitting in front of me at Lord's, hear my friends Kate and Zoe talking about how much they like the cover and buy a copy off me for £20.
* Clearly, this must be the price for the trade paperback, so I am being unfair, but it links to the review and picture of the normal paperback and I am doing a joke. Cut me some slack.