Dvd.co.uk is selling "Sin Episode 1: Emergence" for the PC for £3.99 (inc Del.)
You can get cashback through the usual sites (e.g. 4% Quidco, making it £3.83)
Next best price seems to be £8.73 from TheHutt.
Source Powered Utilizing the same engine that powered Half-life 2, SiN: Emergence gives you stunningly gorgeous graphics and intense physics-based gameplay.
You're Colonel John R. Blade, commander of the most legendary Security Force in Freeport. For years you have waged a personal crusade against the wicked geneticist Elexis Sinclaire. Now, with Elexis at the height of her power, it's up to you to stop her before she remakes humanity according to her own twisted vision.
Explore the most interactive game to date. Create chaos with real-time physics, hack into computer terminals, and experience a living world.
First time player or hardcore gamer, the Personal Challenge System automatically provides you an experience custom-tailored to your skill level.
Brilliant enemies respond to your every move. Shoot, melee, or toss off-handed grenades - your foes will react and keep the pressure on.