You can get from this from PPDirect for as little as £5.49 by using the added £1 off voucher. :)
For those of you who don't know, Gungrave Overdose is a sequel. I don't think you have to worry too much about missing out on the story, the first one only lasted 2-3 hours!http://www.gungraveod.com/
- You might want to check out the official site for plot details, it has a fairly detailed plot summary.
As far as other stores prices go... Amazon.co.uk are the next cheapest at £7.47, not including delivery.
Oh, and I don't think the site wants me to upload an image for this. :D