It may have been posted before I just havent seen it till now, nobody pointed it out and theres a lot of second hand ones going for around the same price.
You don't have to get 2 DIRT's, thats an option yes, but if you like any of the other 19.99 games you'll get both for £30. (they are a bit lame i must admit...)
In-store is where you get the deal, obviously delivery makes it near the same cost as other places. but still cheap so worth a mention..