Saw this in the Pop DVD vending machine at Manchester Piccadilly Station yesterday... they have a "half price" section... some of the DVDs are worth it, others aren't. =p
Assume this will be the same price in other machines, if they're centrally owned / controlled.
Frost / Nixon was either £3.00 or £2.99 (can't remember which, but I guess it's not a dealbreaker !) Have put the higher price in any case.
3 cheapest online seem to be £3.98 Amazon, £3.99 Play, £4.99 HMV