How much "coffee" is there in my cup? Kind of a strange question... 

Suppose you brew your coffee to your liking, a v60 filter or an espresso. The ingredients are water and coffee grounds. But how much 'coffee' is there actually in the end cup? That is, if you would be able to extract the water from the cup; what percentage of the original cup of coffee would be left?

Has anyone ever read anything about this?

What I'm trying to get at is; the strength of a cup of coffee. Everyone uses the saying a strong cup of coffee when they quite often refer to the roasting. This could also be a reference to how diluted coffee is; e.g. an americano. But what percentage of that americano would then be 'real coffee'.
This is discussed in many different contexts many times, I assume. So, I can say not a very strange question in essence.

I advise you to check a previous answer first. It includes a very nice chart and the ideal point for quality brewing of coffee. Also, it shows the extraction ratios, a hint of TDS (total dissolved solids) etc. I copy its link just below:


What&#39;s the recommended coffee to water ratio for cold brew?


As you see, if you can manage to brew an ideal coffee, you can extract only around 20% of the valuable "essence" from the core of the beans. That's the reason why some of the members of Coffee SE tries to re-brew the used grounds for a second time (I assume), as you can follow below:


Brewing with the coffee grounds twice?


Then, of course, some other members discuss the TDS part to find the golden brewing point at the first chart. The easiest way to follow that point repeatedly is done by a refractometer. It is a device that can measure opaqueness. So, by measuring the opaqueness of your brew, you may reason on the amount of dissolved coffee "essence" in the water; therefore you may estimate the strength of your cup.


Options for measuring TDS - Suggested refractometers
Are there any Alternatives to using a Refractometer to determine extraction


I hope these previous answers and the path I've directed you helps you.