If you buy the 3 disc version of Avatar for £17.99 from Play, you can choose another film free from a list. There are a few good ones amongst the dross, so if you wanted Avatar (which is still £16.99-£17.99 by itself pretty much everywhere) it's a good deal.
e.g. you could get Avatar and one of I Robot, Die Hard or Robocop for £17.99 total
P.S. Yes I know that Avatar itself isn't everybody's cup of tea - this doesn't need to turn into another "Avatar is rubbish" thread!