The NGL are components of the gas. During processing at the gas plant, the liquids (butane, propane, ethane, isopropane and gasoline) are processed out of the gas and sold separately. The resulting “dry” gas is sold at the tailgate of the plant. Generally measured and priced in gallons. Liquids prices have fallen because to the increase in shale gas production. The value of the gas out of the ground increases if it is richer / higher BTU gas with liquids. The value of the gas is the the combined total of the dry gas and the liquids. As to what expenses you are being charged and how the costs may be allocated between the gas and the liquids, depends on your producer and state location and the written terms of your lease. If you post the name of the operator, well name (preferably with API number), county and state, someone familiar with the operator and wells in that area will be able to give you more definite information.