The reason why the cost of panel pcb will increase is it lower substrate utilization.
PCB substrate is a fixed size, when your board is small, manufacturers can make panel by themselves, so the rate of substrate utilization is high and less waste area, the cost will decrease.But if your design is panel, the manufacturer must treat your panel as the basic unit, so the rate of substrate utilization is low and the cost will rise. And it needs V-cut and route.
And many different designs on the same board means more engineering data processing and data testing, it will take factory's engineers more time.
Besides, It will increase scrap rate of board.For example,
5 different kinds PCB in one panel, even without V-cut or slot, if something wrong with just one kind board, the entire board has to be scrapped.
