If your mirror fell out it is covered under the general recall they issued on all Canon 5D's years ago. I just picked up a used spare for my wife 2 weeks and ago and as they always have they reglued the mirror at no cost even though it had not fallen out yet. They also install additional metal support braces. Why would they want to charge you to do this? Is the camera modified such as a shaved mirror or something like that.
I took a kind of slow super glue. I know this is not recommended by all because of the solvents used.
With this super glue I glued a 100% silver mirror inside the EOS 5D instead of this 60% normal mirror.