One thing to bear in mind though - a player's 'worth' isn't really an absolute figure. It's what they are worth to any given club.
If Messi joined United, they'd sell 1.5 million shirts and make back 30m in the process.
Is Messi joined Accrington Stanley, they'd sell 10K shirts and make back 200K in the process.
Messi's 'worth' depends on who is buying.
Still, we've already paid a heavy price for Bony and Sterling without seeing much return on that investment. They might come good in the future, or they might not, but 70m+ is still right up there with the world's top transfers, and is this guy really setting the world alight? I don't think so.