I do have a drop checker and it is green
. i adjusted the amount of yeast to produce the right amount of CO2 for my tank. I run it at night so in the morning the thing is light green and then at the end of the day it turns turquoise.
And for the price, if we exclude the equipment, the sugar and yeast is like almost free since i use ~ 1/4 tsp yeast every 1.5 weeks, and 1 cup of sugar at that time. COSTCO sells a fukton of sugar for like $11. OBVIOUSLY, if your tank is larger, proper CO2 levels would be much harder to achieve than in my 10 gallon.
CO2 is not necessary unless you have a more particular plant. You can have a very nice tank especially small if you choose to not run CO2. I did that for a month but the picture of coming home to a tank that has air bubbles on every leaf is so cool. Also, tells me my plants aren't sleepin on the job.