Lego Marvel Avengers (UK)

I was looking for this and it's like £50/£55 on Xbox store, £40 in Argos and whatnot, then I found it was £25 in ASDA and £28 on Amazon. Nearly half the price of the Xbox Store.

Yep, that happens. Physical merchandise can end up cheaper than Digital because a retailer needs it to move. Sitting on a shelf costs money, sitting on a server does not. If the game is there too long, then it costs the retailer more to keep it. So, they reduce the price. This can also stimulate getting people in the door. Come for the $5 game, stay for the $300 device!

On the other side, you will see publishers do Digital sales. All games they publish on a platform on sale at once.