[Anthony]

My suspicion is that spaceships sensors are cheap and heavy because it's trying to make putting them on an interesting design decision within a resolution of 5% of ship's mass, and that's not a very likely situation. In general there's two methods for putting sensors on a vehicle: figure out what it needs and fits within your budget for the role of the vehicle (which will generally result in a sensor weight that is negligible relative to the vehicle), or you're just building a sensor carrier, in which case you don't care that your sensor costs ten times as much as the rest of the vehicle put together.