I've just changed my front pads for the first time at 44k miles, and they still had c.3mm on them.
It's a bit of a pain having to remove the wheel-arch liner to get to the plug for the pad wear sensor (the original sensor broke when I tried to remove it, the heat must make them brittle), but it's all done now....£42 for the pads, £12 for the sensor (Brembo's).
Now I notice that the rear ones are looking a bit thin. £34 for pads, clips & bolts, £7.50 for the sensor.
So, that's less than £100 for front and rear pads and sensors (online auction site).
The disc's look flat and smooth still, with almost no ridge at the edges........good for another 44k at least!

Bob.