I have had my windows tinted (see pics), all windows behind the front driver and front passengers windows are dark, I had this done at the dealer prior to collection, this was £250.
Two weeks later I decided I wanted more windows tinting, I looked for a local tinting business and they came to me, I had the front side windows done, they are lightly tinted allowing 70% of light transfer. I also had a sun strip fitted at the top of the windscreen £130.
It might be more cost effective to look for a tinting business near you rather than the dealer.
There are many reasons for tinting, I wanted glare reduction, UV protection and something to help keep the cabin cooler in summer, some tinting films also provide heat rejection.
As you can see from the outside its dark but inside visibility is really good.
Always check that your tints are legal, I was issued with a warranty card with details of the tint film used.
Hope this helps