Estimated repair cost Canon 5D Mk II Error 20

I have a Canon 5D Mk II that started displaying error 20 (mechanical problem).

What are the current repair costs for any broken parts such as

  • shutter
  • mirror box
  • set of gear

I've received a quote from Canon that they could fix the problem for either € 450 or for all actual costs. Since they can be found for € 500 used, it makes me think that the option with which I should go is not clear.

So what are your experiences, what are these types of repairs?