What would you say?

In my experience, you just can't reason with the people who believe that kind of conspiracy theory. However, one approach I have taken is to personalize it -- ask them to think about the nurses and physicians they have encountered and known in their healthcare experiences so far, and ask them to think about whether they can really believe those individuals would purposely withhold a cure for their illness in order to line their own pockets. I also ask whether they really believe that, if a free, simple, easy cure for cancer existed, it would really be possible to keep that secret, no matter how much the pharmaceutical companies and some others would like it to remain so -- don't they think that somebody, some renegade, some whistleblower, would spill the beans? I also point out that there have been a number of individuals who have claimed to have simple, easy cures for cancer and have done their best to publicize that information widely (a quick Internet search will turn up all sorts of interesting theories and "snake oil" remedies), and yet they haven't gotten v. far -- if any of those cures actually worked, wouldn't we have heard more about them by now? You don't talk to them like you're disagreeing with them, but like you're willing to consider the idea but have some questions about it.

This is basically the same approach you use working with delusional psychiatric clients (and this kind of conspiracy theory could be considered a delusion) -- you don't tell them they're wrong but you gently raise the logical flaws in what they're saying and let them think about that for a while.