Bale is worth £50-60m to a team who can get the best out of him (i.e Spurs prior to Madrid move) but all he's really worth now to Madrid, in this form and what the fans think of him is realistically €40 million at best.
They talked a Serie A team into paying £40m for Higuain and Arsene Wenger into paying £40m for Ozil. Then they talked United into spending £60m breaking the British transfer record for a guy not best suited to English football who'd had one world class season.
You seriously see them selling Bale for £40-60m with his final third production, younger, way more marketable and still the second top scorer in the club after Ronaldo (and would be top scorer at most clubs).
If they sell him it's to make stupid money for the Pogba, Aguero or whoever else fund; not a desperation sell for whatever they might possibly cobble together, like Kagawa or Welbeck. They're forcing him out because they know United are desperate, not because they are.
99% of teams aren't like Barcelona and Man Utd and glad to take whatever they want for players they can no longer accommodate. Real Madrid have grown up in the last few years just like Chelsea. They're two of the better run clubs in world football these days; shocking as that may sound.
They're not going to write off £40m on a guy scoring more goals than United's top scorer, from the wing. When United are desperate for goals, youth and pace. Everyone including Real Madrid know you're forced to overspend. You paid £30m for Herrera and £60m for Di Maria. Why would they sell a 25-30 goal/assist a season 25 year old for £40m?