How does the pharmasist know (or the drug maker, whatever) how much of what to put in a pill? Like say you are using some pain killer. Some are like .05 and some are .07 and on and on. What all do they put in them and how do they know how much of what to use??

Most of a pill is just filler that holds the important stuff together.

What the important stuff is depends on what you want the medicine to do.

They figure out how much of the important stuff to put in based on clinical studies that determine both how much is safe and how much is needed to have a positive effect. Basically, they get volunteers with some disease and try out one dose of the drug on one set of them, another dose of the drug on another set of them, and no drug on another set of them. These studies can take years, which is why new medications take so long to become available after the time they are discovered.

This is also part of the reaon that drugs cost so much. It takes a lot of work to come up with something that may work, then you have to test to makes sure it really does work and is safe, then you have to sell it at a price that will earn you back all the money you've spent on it (plus the money you've spent on researching all the drugs that didn't work out) within a set number of years. After a certain number of years, everybody else gets to use your recipe. Since they didn't have to invest in the research, they can afford to sell it at a lower cost, so you better have made your money back by that point.