Most people in France would sell their data for under €40/month

I suspect a lot of it comes down to how they phrased the question. At this point I think a lot of people have given up on the idea of ownership of their own data, including in the EU. So when you've mentally prepared yourself in a way that you already don't own this thing the value on it goes down quite a bit.

If you were to preface the question with a proposition such as; companies are only allowed to request the minimum amount of personal information in order to business and they are only allowed to retain that online for the minimum amount of time required. Companies that request more than the minimum required or retain it online longer than the minimum required face a 100,000 Euro fine per incident paid to the person whose personal data was violated.

Given this, you could optionally opt to sell your personal data so companies could freely use it and retain it until you opt out. What is the monthly price that you would sell your personal data for so companies could use it anytime they want without request?

I suspect you would get a completely different answer. Where people would now value their data very highly because how the scenario is framed. When you frame a scenario as if your personal data has no value you basically set the market for people in their mind.