Was considering the sale of two of my current vehicles. Both are equipped with CNG. While costly when new and installed, received some federal tax incentives. Now upon looking to sell the truck and SUV, was curious as to how much, if any, value this would add to the market value. Both units 7-10 years old. Yukon and F250. Any info appreciated.

I'd say it is up to the person buying it.
Around these parts it would be a HUGE no-no since there are very little places to get it from so the price would be a give away.

I'd say list it and see what the response is and go by that.
If you don't get any calls then you know you need to give it away, if you get flooded with calls then you can ask a premium.

My thought is that the vehicle *could* be worth the cost of the CNG kit, above market value, given that both vehicles are stand up condition and reasonable mileage.

They're worth what someone will pay.