It is true that CNG high pressure hardware (tanks, regulators, fuel lines, etc) is very different and more expensive than the equivalent LPG hardware. However, CNG and LPG systems are very similar operationally and use similar mixers or injectors.

Also, LPG is also a by-product of natural gas processing in addition to crude oil refining. HD-5 Propane is the LPG recommended for motor fuel applications and contains a minimum of 90% propane and maximum of 5% propylene.