I'm sure he'll take some advantage of that, but I don't think the difference will be as big as you think, he doesn't have one of those money-hungry agent (clearly given the contracts he's turning down), he doesn't even have an agent, and his family is reasonable. I think he'll want to have as strong of a team around him as possible. Any deal of this size would be in installments anyways, so even 200m is not 200m taken right away, but it's a big chunk and that signing on fee is another big chunk regardless of what Madrid paid to PSG, that's more upfront costs that take away from the ability to make moves in the coming transfer windows than if you take a bigger hit on his wages and signing on fee a year later I would imagine, but I'm no expert.
I see your point, but I don't know if I agree with the numbers, and if it does work out and Mbappe goes to Madrid for a free, I think they'll be much better off than if somehow this deal gets done in the next hour and a half.