Cheapest I've seen these. Killing Joke is easily one of my favourite Batman graphic novels made into a film. They do a great job with this, and there's like an additional bit which 'expands' on the original novel at the beginning.
Animated adaptation of Alan Moore and Brian Bolland's graphic novel featuring the voice talents of Kevin Conroy and Mark Hamill. The story follows Batman (Conroy) as he tries to apprehend his nemesis The Joker (Hamill) while exploring the origins of his most famous foe. After escaping from prison, The Joker targets the Gordon family for his next attack and kidnaps the Commissioner (Ray Wise) to prove that a bad day can turn even the most honourable man insane, just like it did to him. With Barbara (Tara Strong) caught in the crossfire, The Joker taunts Batman from afar as he tortures Jim in one of Gotham's run-down amusement parks. Can Batman stop him before he succeeds in driving Gordon to the brink of insanity?
*Both a part of the 3 for £20 offer (as is Batman Vs Robin (based on 'Batman: The Court of Owls' by Scott Snyder) and a few others - so you can get three cracking Batman films for £20)