Good point, probably suggests a 35-40M fee and not a 55-60M one. Cavani only got 210 on a free so probably going rate on a free for Martial is probably 50-75K or so less than his wages, which is 4.5M a year difference which over 4-5 years is that 20M difference.
Not for Manchester United. But like I said with Lazio and Immobile, there is probably a team out there for whom he can move to and stay at his wages. Roma is paying Mkhitaryan 180 a week apparently and they will be rid of Kalinic's 70 and Dzeko's 150 soon enough up top, and Martial could go down a fair whack and still make the same amount because of their 21% vs 45% rate since they have a Beckham tax there now.
The question is if there's a big team like PSG that wants him and if not, is there a Roma/Leverkusen/Sevilla who wants to build their attack around him for the next few years. My guess is yes, one of those is out there. He's 25 and has obvious skill and 25 french caps and is 15th in Prem goals among active players. Domestically, it wouldn't shock me if Arsenal were interested. None of the other teams with wage earners near that high make sense, though Liverpool might have if they hadn't signed Jota this past summer though obviously yeah, no.