Why so much money? Only a fiver on steam...

I think GoG, Steam, and Humble Bundle have their own separate sales rotas; there's 66% off on Steam at the moment, and also some (rather expensive but apparently good) bunch of games that include CR at the moment:
http://store.steampowered.com/bundle/1576/

I think the game has been on sale on GoG before, I guess it will be again; OTOH I've spent going on 1000 hours in the game, so I felt like £15 was pretty reasonable:)

At the moment (winter sale) it's $6.69 here... I would say that's more than reasonable for a full modern game with frequent updates.