If you consider CNG in terms of GGE (Gasoline Gallon Equivalent), a vehicle should get roughly the same fuel economy on CNG as it would on gasoline. CNG Fuel Economy of OEM CNG vehicles appears to be slightly less than their gasoline counterparts. Enbridge in Ontario uses ±5% as their rule of thumb for CNG fuel economy relative to gasoline. Even if CNG fuel economy was only 80% of gasoline fuel economy, CNG is still cheap, especially if you get it from a gas utility. For fleets, there's a federal CNG tax credit on fuel purchases which makes it even cheaper.

A typical CNG Injection Conversion runs about $6000-$7000. Aftermarket CNG conversions do not require any special engine components other than the engine should be in good mechanical condition. Engines running on unleaded gasoline already have hardened valve seats but upgrades during a rebuild never hurt. Tuning the PCM to optimize timing for CNG would help but would also be considered tampering. There are also special CNG O2 sensors that are more responsive to CNG.