There is, really, not much to choose between the prices of the different makes of paint apart from the fact that beautiful, original decorator colours often cost more than the ordinary standard ones, and that chain-store paints can be the best buy of all. Even having a gallon of emulsion (which covers a lot of wall) mixed to the exact colour of your choice usually costs very little extra.
Wallpaper is something else entirely; costs vary enormously from roll to roll. One of the cheapest ways of covering walls is to use ordinary lining paper; this can be bought in rolls.