I think things like this should be taken into consideration when talking about games with legs...
http://darkzero.co.uk/game-news/madworld-already-dropping-below-20-in-uk/
Madworld already dropping below £20 in UK
"In what is likely to be some very bad news for Sega, although good news for us, Madworld has started dropping in price at many retailers across the UK - a sure sign that stores are having trouble shifting their stock.
This week it seems many outlets have saw fit to slash the price below the £20 mark. The price cut comes less than two weeks after the games launch, when the game was selling for around £29.99 - although the RRP was £34.99.
Currently the best price seems to be £16.99 at Gameplay, although £17.99 at Gamestation and GAME offering it for £19.99 are good prices too."
(more at the link)