With Bale, he's had two seasons, and CM, I know you love saying this isn't his only great season, but it was. There is a difference between improving and being a great season. 20+ goals is generally indicative of a very good season, 30+ is great and 40+ is outstanding.
65mil + is finished article pricing, 25-30mil is what he might be worth today, you add to the 25-30mil based on his chance of being the next Ronaldo, and considering the number of guys who score 25goals one season, then never break 20 again hugely, hugely outweighs the number who stay above 20 a season and the number who go on to score 40+ a season is so utterly miniscule, that factors into the price. 40mil is WAY too much for Bale as yet on a single 20+ season. Ronaldo's cost was because he'd proven a huge goal scorer, beaten the 40 goal mark in a season AND still had potential to be even better. I'd say without hyperbole that 99%+ of players who manage a season with over 20 goals don't go on to become 40+ goal a season players, and a pretty small number 5-10% manage to be 20-30goals a season consistently.
I think one more season over 20goals would make him a smaller risk and increase his value, but it would take a 40+ goal season to warrant a 65+mil price. You can currently buy better goal scorers for less cash, RVP was 23mil and outscored him again. Cavani, Falcao, all have outscored Bale and have done it season on season for multiple seasons.
Lets also point out that he scored a significant number of VERY similar goals, in a way that defenders will be on the look out for, and keepers and midfielders. He may not have been considered the biggest risk last year for Spurs, this year he will be(dependant on which strikers they buy) and as such people will target him, fouls, physical play and simply looking for how/when he likes to shoot and putting men in the way. There is a reason that strikers who have a break out season then get targetted by the opposition(fairly and unfairly) and many/most go on to have worse seasons.