At the moment you can get it from Amazon for under £16, which is pretty cheap for now.
But if you want it as a direct download then Steam is probably the best place, but it costs £30 from there.
If those are too expensive, your best bet is to wait for a discount. Steam are known to have a lot of sales, and over time it'll probably go down in price on Amazon too. I recommend you keep an eye on it for Steam; sometimes they just put things on sale for 50% or 75% off or something!
