The answer is that there are many answers. I can verify from having worked for various companies as well as having the opportunity to speak to many that the answers are fairly well represented here.
There are those who carefully price their coffee to be able to sell at the lowest possible price, those who try to extort the highest possible price, and everything in between.
To answer what I think your direct question asks: why are 12oz bags (from "third wave roasters") usually about $18-20 USD?
I think this is the price that the market currently supports for "third wave roasters" to sell their products. I am thinking of roasters like Intelligentsia and Counter Culture.
I think there are many roasters who imitate what these roasters do (directly support the origin, train their staff to professional levels and pay developed country wages, and continually improve their processes, etc.) who piggyback their pricing to varying degrees of success.
The sad truth (in my opinion) is that consumers can not yet tell the difference between roasters like Intelligentsia and Counter Culture against many third wave roasters that give the appearance that they are the same thing. I think this is getting better though.
In the meantime, I would agree that many (third wave) roasters charge an insanely expensive price. About half the coffee I order in a year (because I like to try new roasters) is total crap and I find myself duped by roasters pretending they know what they're doing. They may buy a beautiful coffee but ruin the roast.