Well if its a signing at the megastore you can virtually guarantee that will be the RRP of £18.99.
Book signings usually are at full price cos you get the increased value of someones signature in it (I got Charlton Heston's autobiography a few years ago, signed, and of course now he has passed away the value of the book increases)
Amazon is cheapest at the mo for £10..........so get it virtually half price and get MLT to sign it another time (at an evening match for instance as he is usually there when not on Sky)