Koch Bros motives are far from altruistic, they have vast chemical and paper industry holdings that are huge nat gas consumers. Pipelines are actually neutral on cost of nat gas. They are levered to volume of gas moved not necessarily the costs.
As far as the payback, if Congress passes a law requiring that a mcf of cng be taxed at a given rate the pay back is a pretty certain. The benefits achieved building out the infrastructure and the multiplier effect achieved by putting people back to work might be more debatable.