What you have to remember is that there are different resources available depending on different players. As has been mentioned a number of times, Messi is a marketing phenomenon and will open up additional sponsorship income that wouldn't otherwise be there. Hence, it isn't a case that we could spend £400m on other players fees and wages because, if it were other players, we wouldn't have the £400m in the first place.
This would be a one-off deal for a one-off player. The tricky bit is ensuring that the rest of the team understand and buy into that point of view.