Application is the issue. The Volt and i3 were passenger cars with little to no payload or work duty requirements. As such, either of those cars could be pushed down the road with a 60 hp 3-cylinder motor sufficiently. In the volt, at highway speed the electric motor actually turned off and a clutch connected the ICE directly to the wheels as that was more efficient then generating electricity. Plus, the Volt was sold at a pretty significant loss to GM.You don't need a full-sized ICE. While obviously lighter and smaller, the Volt (1.5L 4) and the BMW i3 (0.65L 2-cylinder from a scooter) use small displacement engines to generate power for the electric motor. The 1.5L in the Volt gives it a total range of 420 miles (8.9 gallon fuel tank) and an MPGE of 106. So... no it's not double the cost for zero benefit. Especially since most of the time you're running on electric power only and using no gas at all.
In a truck or Jeep application, the load requirements are exponentially higher than a passenger car. Big tires, loaded down heavy, or towing, there are times we are at WOT asking for everything the little pentastar can supply. For instance, running 80mph down the highway requires double the horsepower and fuel as running at 50 mph. Just how big and heavy of a battery are you going to install to try to make up the difference? What happens when the battery runs out? Are you willing to drop your speed down to 50 instead of 80 until you get to your destination because that's all the horsepower your "range extender" makes?