I'd say belt change would be more than that - I had a mate do my V6 A6 and that came to £600 at mate's rates - that included £450 in parts. Audi quoted £1200 which didn't include the water pump or thermostat change that are highly recommended to do.
Just checked the paperwork for my S8, which had a belt before I bought it, and it came to £820 but they did not change coolant or water pump, or thermostat in that price
