Relative costs of this project:
Sensor Development - $40M
Sensor Production - $20M/Year
O&M Cost - $10M/Year
Deployment Cost - $2/sensor
An adversary forced to spend all their time turning over rocks to make sure they aren't sensors - Priceless.
The Border Patrol uses hidden sensors along the Southern border. What the opposition does sometimes is come along with metal detectors and RF sniffers to locate them. It's a constant battle of adaptation to keep ahead of their countermeasures.