This is called a "series hybrid" rather than the more common "parallel hybrid" (eg Toyota Prius) and has been around for a while, including the BMW i3 with range extender (and London buses, and various other vehicles!). It's more expensive largely because the battery pack needs to be sized much larger to be able to provide enough current for all propulsion / regen. On the other hand, the combustion engine can be undersized and run at its most efficient RPM continuously - the BMW range extender is a 600cc little scooter engine that can provide enough power for continuous highway driving.