I don't think it'd be TOO unfeasible. United paid £34mil for an unproven left back. We'd be paying £50ish for someone who could win the Balon Dor. Plus we'd get so many Colombian fans as he's their golden boy, which expands our fan base and marketing potential in South America and gives us somewhere else to scout. Besides, if he's unhappy, Monaco has to take account of that, like we do with Yaya.
Best player at the World cup, most goals and assists at the world cup, can play any attacking position, young, strong, direct, most assists in the league last year in an average team, a decent amount of goals, most chances created in the league (I think, certainly one of the highest), consistent in two leagues and the national team. Maybe wait for the hype to die down, but if any team is bidding for him, we need to at least match them.