Mirror cleaning on Nikon D7000?

I have a D7000 and when you look through the view finder I can see black marks in it. These marks do not appear on my images which is a relief but it's so annoying and off putting having them there.

I think from looking round on the internet that the mirror maybe dirty or scratched. I contacted Nikon but they can't quote me unless they see it. My question is does anyone have experience of this? If so how much roughly will it cost foe the mirror to be cleaned or replaced?

I think this happened when a "helpful" friend cleaned the sensor for me. At that time there was a black mark on my images but that has been cleaned away.

Actually it's far more likely that it's the focus screen. Mirrors are so slick not much dirt sticks there.

Contact a pro level shop in your area, shouldn't cost more than $50.

Plus one on the finder screen, if the mirror were dirty you would not see the dirt since it is not in the plane of focus.

And if it is just the focus screen I would not pay to clean it until the camera goes in for sensor cleaning or other service issues.