How expensive do prescription drugs need to be to fund research?

Not at all if research is publicly funded for the public good

The answer will always be, "more expensive than they are now". It's not like there will ever be a point at which more money, if available, could not in any way be used for some kind of drug research that might be useful. It's like saying "how much money do you need in order to keep from ever dying?" There will always be health problems, and there will always be some at least semi-plausible way in which we could spend money to try to research a way to at least ameliorate the condition.

So, this doesn't mean we shouldn't spend some money, or even a lot. But, it will never be enough, so at some point we will have to say, "we could spend more, and it might give us a little more life, but let's not."

I don't think we're remotely ready for that conversation.

This is such a complicated issue, I'd highly recommend the following as a primer:

https://www.forbes.com/sites/matthewherper/2015/10/13/four-r...

Also Derek Lowe is invariably awesome in this space, one highlight:

https://blogs.sciencemag.org/pipeline/archives/2019/02/05/ta...

Drugs occasionally need to be expensive, particularly on-patent drugs, to make the endeavor worthwhile. That said, it's a two-way street. Making drugs insanely expensive (as has been happening more recently) is breaking the implicit trust that those in the medical field must work hard to maintain. Traditionally pharma was careful not to abuse the inherent "your money or your life" dynamic, it seems these days that is breaking down. It's sad because for very powerful drugs (i.e. gene therapy treatments) the cost is actually quite high to even make the drug (I've heard quotes in the 100's of K even in the 1-2 M), which means the treatment has to be expensive. That said we need to work hard to bring that down, and there's absolutely no place for the kinds of shenanigans going on with insulin among others:

https://www.nytimes.com/2019/01/18/opinion/cost-insurance-di...

With abusive practices like this, how can you ask patients to trust you when you say, "yes but this time I actually need it to be this high"?

How marketable do prescription drugs need to be to have their R&D funded?

Doesn't this boil down to Citizens United - if corporations are allowed to use money as they see fit because money is roughly equivalent to free speech then all three branches of government are responding to the entities with the most of it - in this case big pharma.