Hi Gareth, I use Audi's Flexible Service which means that they only see the car every two years. The cost of my latest service was £399 including parts. Viewing this as about £200 per year it looks quite reasonable. Also I know that I am getting genuine Audi parts.
Back to cambelts, perhaps the question should now be, "How long will a water pump last?" Then the advice would be, change the cambelt when the water pump is changed. At least a failing water pump won't spell instant disaster for the engine.
For Audi to now say, across their entire range that uses such belts, only mileage counts, they must have confidence in them and I expect that is backed by extensive research, advanced technology and user feed-back and warranty claims. Audi UK seems now to be aligned with Audi Germany.