Hi there,
I think a lot of the String Quartets charge in the region of €600-650 upwards for the ceremony. If you have the budget for a quartet, I agree that they can be a nice addition.
If your budget doesn't stretch that far and you would still like some classical music at the church, I can offer a similar musical sound at much less of a price. I accompany myself on piano/keyboard and integrate nice full string sounds for some of the classical pieces.
Here are some recorded samples (played by myself on keyboard):
Pachabel's CanonAir On G StringBitter Sweet SymphonyIf it sounds like something that might interest you, I'd be happy to give you a quote!
All the best

Keith.