Following on from Essell's points, these hugely inflated sums of money for 'works of art' are situations brought about by art dealers. The likes of Saatchi etc ensure that very rich patrons who want to be thought of as discerning want to acquire the latest / best / most significant, and want to be seen paying a huge amount for it (or to remain Importantly Anonymous).
I think this particular painting by Munch is very interesting and certainly much better than his other works. It also sits in an interesting point in the history of 19th and 20th century art. But I can't even get my head around £74M. Thank goodness it's all over t'interweb so we can look at it for free.