The whole point is to sell more cars, the EREV Lightning will not be cheaper to produce or sell.
At current prices, the standard range battery in a Lightning (which is nominally about 107kWh or so) should cost under $10K. They will not be able to shrink the battery enough to offset the cost of putting in an engine and generator. For one, they have to stay competitive with the Dodge EREV pickup, which will have a ~90kWh battery.
My guess is they leave out one or two modules from the standard range pack and price the truck starting at $70K. They won't make a ton of money, but they might be able to get a nice boost in volume to make up for it.
