Multiwinia + Darwinia, PC - £13.28

If you pre-purchase Multiwinia on steam, you get the fantastic Darwinia for free. Comes to £13.28 by my maths.

If you haven't already got/played Darwinia, I highly recommend it. It's a wonderful, unique, beautiful game. Very indie, but at the same time, very polished, and it has a great sense of humour (be sure to download the PDF of the manual too).