Problem is that I don't think it would be a case of just another £20 or £30 - you've got all that extra metal (or carbon fibre if you will) - then you need the extra lift, so larger wings, then more fuel, and more fuel to carry the extra fuel, etc.
Besides in time the airlines would only think "well we can lop those 3 inches of all the seats and put another column in" so the benefits would be short lived...
Short of legislation, I doubt it would happen