Oh. In that case I did not (and still do not) understand the rule. Carry on 
This is more or less how it was explained to me...
There are Sensor Housings, Sensors, and Sensor Functions. Sensor Housings include not only the Wall-Mount and Handheld housings but also vehicles, drones, armor, and weapons. You put a Sensor in a Sensor Housing... this includes Single Sensors and Sensor Arrays. The rating of the Sensor determines how many dice you get to include on your skill check when using that sensor (pg 445) and as the cost is based on the rating, this also determines the cost. A Sensor Array can contain up to 8 Sensor Functions no matter its rating while a Single Sensor can only ever have one Sensor Function. Sensor Functions (Camera, Atmospheric Sensor, Microphones, etc...) are free because you've paid for them as part of the cost of the Sensor (Single or Array) when you bought it. Things you add to those Sensor Functions (adding Vision Enhancements to a Camera for instance) you do have to pay for... all you get for free is the basic model.
Oh, and make sure you pay attention to the Sensor Housings table at the top left of page 446... this table shows the maximum rating Sensor you can put in a particular sized Housing.
And yes, of all the things in 5th Edition... this is the one thing I think they did better in 4th.