About 5-10 million Dollars each
, judging by people's willingness to risk their own lives. I know it sounds heartless at first to attatch a dollar value to a human life, but all other workable approaches to policymaking work worse. This is why health insurances, traffic engineers and other people who work to protect lives use this approach in designing their policies and technologies.
But to return to your point, those people didn't die because it was hot. Places like Spain or Southern Texas are routinely hotter than France. These deaths occured because temperatures were higher than the French expected
when they built their houses. There are two ways to prevent this from happening in the future: you can make temperatures lower by reducing greenhouse gas emissions, which is what you are proposing. Or you can adjust your expectations, expect higher temperatures in the future and behave accordingly. For example, the French could install air conditioning, improve their medical infra structure, et cetera. This is what I would do about it, because I believe my approach yields better life protection for the money.
Another point worth noting is that heat waves happened before global warming too, so it's imprudent to assume that none of these 15000 people would have died because
of global warming. My understanding is that much fewer people are dying from extreme weather conditions today than did 100 years ago. I'm not saying we have have global warming to thank for it, but it also didn't prevent it. That's more reason to believe technical fixes will do an adequate job.