Most of the cost of a jump point sensor is going to be the cost of running a ship to place it, and the time spent by the player doing so. Since you just need to be able to detect a ship making transit, if the sensors are close enough they do not need to be very big. I suppose at high tech you have to worry about stealthed ships jumping as far from the jump point as they can, but most of the time, the detection a jump point sensor is going to give is the notice of their destruction.