In Arsenal it mentions how you can upgrade Sensors by upgrading each individual bought sensor. But, some sensors don't actually have a cost based on Rating, or even a Rating at all, really. Are those basically freebies when upgrading sensors, or is there something I'm missing?

Yes, those are basically free. It can even be exploited to some extent, though I advise to take the prefered sensor suite regardless of cost. It´s only a few hundred bucks.

Said rules doesn't really work out though, as it contradicts the main book.

The Sensor Attribute is the Intuition Attribute of Vehicles - to be used for Perception Tests with Sensors that don't have their own rating, like cameras. (Vision Enhancement on said camera is a bonus to that test, not the Rating of the camera)
So you can't really upgrade the Sensor Attribute that way...

It's also funny that an Emotitoy for 100 bucks has Sensor 3 while comercial vehicles are at 1 or 2...

FUBAR nicely sums up SR4's sensors, and by extension, vehicle rules.

All Drones or Vehicles come with (or some combination of):
Atmospheric Sensor
2 x Camera
2 x Laser Range Finder
2 x Motion Sensor
Radar

This is automatically included in their purchase.

Of these only the Atmospheric Sensor, and Radar have ratings. I have ruled that the cameras have a Vision Enhancement which accounts for their rating, I don't give them audio enhancement, because I don't feel that cars have a definate need for audio in sensors. Only when making a general sensor test do you use the sensor rating of a vehicle which is the average of all sensors in the vehicle. If you are using a specific sensor then you use the rating of the device.

Sensor Rating of vehicles has no use beyond your original purchase of the vehicle. Once you modify sensors, upgrading, or ripping out the sensor rating for the vehicle will no longer apply. All it provides is a short hand way of telling the rating of the standard issue sensors on the vehicle. You'll notice that no device has a sensor rating higher than 3, this is because Atmospheric Sensors (and Vision Enhance w/ my rule) cap out at 3. So even though emotitoys have a sensor of 3, the only thing they can possibly have at that rating is an Atmospheric Sensor (or Camera), since they can't have Radar due to space. Smaller devices are likely to have higher ratings for sensor because they can't put as many sensors in them so they put higher grade sensors to try and compensate.

Yes you can rip everything out of your car, and just have a rating 6 Radar. This would give your vehicle a sensor rating of 6... But it would only be able to do anything where radar would be the appropriate form of surveilance.

Sensors and EW both need to be totally redone, IMO. Sensors is such a mess that it's laughable, and Electronic Warfare and Jamming, as it is now, allows for simply bizarre stuff like jamming an optical camera with what's presumably a barrage radar jamming equipment setup. They should just grab somebody who knows what they're talking about (WMS comes to mind) when it comes to EW, right about the same time they rebuild the Matrix rules.

Most Drones will never have Radar, as it takes 5 slots of their sensor package.

So, that brings the tally of things that need to be fixed/added by Unwired:

-Hacking large systems
-Agent/Doctor Smith
-Programming rules that don't suck
-Programs/hardware above rating 6
-Electronic Warfare
-Sensors
-Technomancers
-Script Kiddies (Hackers w/ Logic of 1 being god in the Matrix, but suddenly losing the ability to use computers when they leave)
-Microscopic Vision and other cybereye/ear mods being available as non-ware equipment
-Retransmission units that *aren't* mobile

A tall order, I'd say. Anything I missed?