Cristiano isn't going to slide into oblivion anytime soon. 50+ goals for the current season (his third most productive season at Real Madrid, just short of the second placed 55 goal mark in 2012/ 2013) doesn't half suggest the onset of decline. He has rather cleverly learned to optimize his playing style to reduce wear and tear, and there's very little wasted movement with him these days. Add that to his crazy determination levels, plug him at #9 to further reduce the workload, and he could play at a very high level (30+ goals per season) for another 5+ years. He's almost 1 year younger than Ibrahimovic when he moved to Paris Saint-Germain, and the Swede has been exceptional value for money. Ronaldo has the drive and quality to go above and beyond that.
As for the transfer fee, Ronaldo is one of the the biggest individual sporting brands in the world, and arguably the most marketable footballer around. His return would be met with immense fanfare. Combine that with Manchester United's massive global support base, and Woody's commercial expertise - and we'd recoup his fee even before his contract is up. Even if there's a minuscule change of him leaving Madrid, we should definitely explore all possibilities.
Not sure he will be a negative presence in the locker room either. Real Madrid's circus atmosphere brings out the worst in people. Yes he can be petulant but a lot of Ronaldo's actions are indirectly derived from the intense scrutiny and fan abuse every one of their player has to face week in and week out. United provides a more welcoming atmosphere with Fergie sitting in the stands and the likes of Giggs in the dugout, who both knew Ronaldo when he was a relative nobody. United lack a consistent clinical threat in the team and he is more than capable of filling that void for the medium term. Plus he can be a training pitch mentor for the likes of Memphis and Januzaj. Not sure if he will leave though, there's still some unfinished business at Real Madrid.