I would sell him, the chances of him having a season like this one in the future are uncertain. In purely football terms.
But this seems, if true something of a power play by Madrid, attempting to re-establish themselves as the biggest club in the world. As mentioned the £100 million would mean much more than the signing of one player. It would be symbol of their dominance. Just as Zidance, Beckham, Ronaldo et al were in the past.
It's undoubtedly bullshit though, just another in a long line of hugely exaggerated estimates of the value of Cristiano Ronaldo (I still think the Ronaldo when hear that name alone).