Oops, you are right. However, that $5 sugar/yeast CO2 won't last for few months.
Couple of weeks top, assuming we are talking about just one bottle. Generally, in a larger tank, something bigger than 20g, the cost is no longer cost effective.
He has a ten gallon, and I actually used to use DIY Co2 on my ten gallon too. I bought a $4 bag of sugar and I only used a quarter in the month and a half I kept the Co2 going. I had a very high bubble count also. I think that too many people are scared to use DIY Co2 because it is "obsolete." For a ten gallon tank, I believe that changing the mixture every two weeks isn't so bad. The only reason I stopped using Co2 was because I got tired of having to constantly trim plants, which is pretty evident that it worked well for me. Don't get me wrong, if you have the money, pressurized is the way to go. It's precise, constant, and easy. But for tanks under ten gallons or for just testing out Co2, DIY is great!