Unfortunately, we're paying a premium for a player who is pretty short supply of.
As Manchester United, in this already inflated transfer window, dealing with a selling club who don't have a neccessity to sell at this late stage, I'd suggest £80M is about right?
The original poster's (couldn't find it unfortunately) comments regarding Ronaldo's sale in 2009 failed to factor in inflation.
In a normal line of business (i.e. not elite football), Ronaldo at that point in his career would go for c.£120M today. More realistically it would be £140-150M.
Similar comparisons can be brought for other players this potential transfer is being compared to.
Personally, the only comparative stats that would help to provide a half decent insight into Antony's potential would vs a similar player, at a similar age who has followed a similar career path to his in the past 5 year's (Brazil > Holland > EPL) and who has played CL football.
So many variables when comparing player for player across different leagues.