This is one of the best value games on the 360 with 5 Half Life games in one great package and received some of the best reviews on any of the current generation of games on any format.
It's now available for £17.93 including free delivery and you can also go through Quidco to receive an improved 10% cashback as well but it states that cashback may be rejected if you use a voucher (there's a 5% one, so it's upto you if you want to risk it)
This is almost half the price of the second cheapest retailer at the moment and with a bigger cashback offer too.
Hope that helps. :)