Are you referring to the focus screen, or the electronic display?
The focus screen is end-user replaceable (Canon sells optional focus screens and you can put them in yourself.)
I don't know what Canon charges to replace the electronic display. You'd need to contact them.
EDIT: Nevermind... I just noticed your other post that it's the LED brightness issue -- not the focus screen.
Also, I still have a 5D II and I'm checking all the menus -- there is an adjustment for the rear LCD screen brightness, but I can't find one for the LEDs in the viewfinder. I thought perhaps altering the LCD brightness might also affect the viewfinder, but as far as I can tell, the brightness in the viewfinder doesn't change regardless of what I do to the LCD brightness.
With regards to having your EOS 5D Mark II serviced for this display issue, we would recommend setting up service on our website. Only when we have the camera on hand are we able to provide an evaluation and a price for the repair.
Click here to be taken directly to the EOS 5D Mark II service and support web site.