In theory it should be cheaper to fly, there's much less going on mechanically in a fixed rotor and there doesn't seem to be a pilot. However, I'm pretty sure that people have shown that creating human-scale quads is actually impossible with current battery tech. So, IMHO this is just another design that doesn't have anything to do with what is possible in the real world today.
For a quadcopter you need very fine control of the rotors, making a combustion engine hard to use. So then you will need to do combustion -> dynamo -> electricity -> electric motor.
A helicopter is controlled in a completely different way.
Larger multicopters are free to use alternative control methods. The method used in toy multicopters is both inexpensive and cheap yet other options do exist.
