I don't think is a deal as it's more of a lucky find. My local ASDA (Liverpool, Breck Road) has recently been stocking really old 360/PS3 games and selling them off between £5 and £10.
While I was having a look today. I noticed this in the Xbox 360 sale section: http://i64.tinypic.com/xbg42b.jpg
The box says Call of Duty Ghost Season Pass. At first I left it on the shelf as I have no interest in COD Ghosts. Until I noticed it said £35 at the bottom. After having a read at the back of the box it says you don't actually get the COD season pass but instead you get £35 credit to use against the season pass or anything else on store. So I bought the last 2 they had.
After opening the box I can confirm no season pass was in the box. Instead there is a £10 XBL voucher and a £25 voucher. So I got lucky and managed to get £70 Xbox/Windows 10 credit for £20.
Not a lot of stores will have these in. But might be worth checking your locals sale section.