This might be the cheapest I have seen yet for series one of Battlestar Galactica (thanks to potlepa). It is priced at £15.99 from CDWow, but by using a combination of discount links and vuchers, you should be able to get it even cheaper... at least for £14.99 or possibly knock £3 off the price. I'll put a link in the forum thread to Gary_Rip's CDWow maximum discount thread.
Details : Edgier and racier than its predecessor, Battlestar Galactica 2004 is a complete re-imagining of the 1970s series - upping the ante on the action, adventure and drama that made the original so popular. The groundbreaking story of man versus machine remains, but this time, the Cylon robot is no longer a recognisable enemy... it can move among man.
With the 12 colonies of man virtually destroyed in the climax of a hundred-year war with the Cylon empire, President Roslin (Mary McDonnell) and Commander Adama (Edward James Olmos) gather up the few humans left on the twelve worlds and embark on a journey to find the mythical planet Earth, the supposed thirteenth colony of mankind. Battlestar Galactica 2004 gives new life to the stories and ideas that were at the heart of the original series, with a compelling plot, breathtaking special effects and riveting, action-packed adventure, speaking to both long-time fans and a new generation of admirers.