12 views and no replies. I'm feeling bad for you :'(
I don't know if service costs are different in the US, but here's my experience:
I got a full 6k service + belt change for not much over $300. That includes valve adjustment, oil change, hydraulic fluids, etc. So 300 Euros for just the belt does sound pretty high to me. I have never had just that service done though, so I can't give you a US price for it.
I would expect the belt change to be an hour of labor plus parts. It's really not too involved.