Ronaldo reportedly earns around 900k p/w.
Hypothetically, if we were willing to match that for - what - a couple of seasons, the deal would be massive based on wages alone. We'd also have to pay Juve a transfer fee (his contract doesn't expire until next June), and even a "reasonable" one would surely be...what? 30-40 mill?
To put it in perspective, Ronaldo's wages over two seasons would likely amount to about what Dortmund are apparently willing to sell Sancho for (Covid discount and all).
It's a lot of feckin' money, in short, for a 36 year old player.