[UPDATE]If you own Limbo on Steam you can get hold of Inside for just £8.99. Just add it to your basket and the discount will appear at the checkout. Cheers to blooo for the heads-up!
While it has been cheaper, this is the best price I can find for this one at the time of posting. I've been meaning to get hold of this one, as PlayDead's follow-up to Limbo has been heavily praised for its challenging puzzles and interpretive storyline.