I got the complete HHG on Kindle for £2.99 at one point last year. I already have all of the print books but it's nice having the e-book version so that I can dip into it Guide at random points when I've got nothing else I want to read.
Kindle books do bounce around in price a lot so you just have to keep your eyes open for the deals. I almost never pay more than a fiver for an ebook.
I never pay more than a fiver either,
i have never read the HHG, I have listened to the radio versions, tape versions and watched the TV series and film, but never read the book until now.