Sainsburys have season 3 and 5 in the shelves in my local sainsburys for £15 each. Cheaper than online from what I could find.
I'll let you guys decide if it's a good deal or not.
Not sure about season 2, 4, or 6. Season 1 is £24 which obviously isn't a good deal but if you love 24, chances are you have it already.
I know that boxsets are cheaper in total but this is good for those are missing certain seasons.