A lawn meter reads just outdoor usage and there is no sewer fee. Although it costs approximately $1,200 to $2,000 to purchase and install, it should pay for itself after 2 years. For long term home owners this could be a huge cost savings.
"In summary, if you are using a standard water meter, you pay a sewer fee for water you use, plus the water that goes down the drain. For many homeowners, most of their water never actually goes down the drain. Instead, it’s used to water their yard and other similar uses. But whether it goes down the drain or not, you are still charged the sewer rate. "
Another FYI, if you want a lawn meter, Houston homeowners must apply through the city.