For that to work you'd need to use very long exposure of several seconds or even minutes with camera on tripod. While certainly interesting method, using a drone for this would pose a several challenges. The major one is the motion blur. Drones...
I think ND filters are useless if you plan to shoot stills only. Camera sensors are biased to assume an overall scene reflectance of 18%. If you put on an ND filter. the camera will simply interpret the scene as too dark and will adjust shutter...
Not only that but sometimes it is on a special. I got it as a bundle with the Speed Editor Panel for the price of the Speed Editor, so the Studio was basically free.