An inverted measuring cup, smaller for lower flow CO2 flow rates, then run it for say 1-5 minutes= mls of CO2 gas per min or 5 mins etc.
This is GOLDEN.
The mls per min can be converted to seconds if you wish or mls per hour etc.
This will be the SAME no matter what type of CO2 system you have or bubble size etc etc.
So this in effect, standardizes ALL CO2 flow rates for comparisons.
Cost? Most already have a measuring cup, if you want, you can use a Class A kimax measuring graduated cylinder etc.
So the cost is basically Free.
Easy of use? Timer and wait. Easy.
World wide, EVERYONE can use this simple method.
But........does not say what your CO2 ppm, but..that's a relative measure and target anyway. So this gives a good way to measure the CO2 input.
A pH meter is the best method to measure relative CO2, I would never suggest a drop checker(DC). DC's are cheaper for sure, but hard to read and hard to respond to change in the tank and very hard to decipher 0.2 pH units, you have too much(yellow), too little(blue) maybe okay/good(greenish).
I can look at plants and fish and get better more accurate information.