Hi,
I did an intensive course for mine, and there were a few people doing the same. I'm under 21, but they were doing the DAS, and looked the same.
If you want to know more about the part 1, there's a good page here: http://1stclassmotorcycletraining.co.uk/New%20Test.html (Great centre btw, if you're in the area)
I personally found it quite fun, riding around a massive great pad, on ultra-grippy tarmac doing slaloms between cones... But asKirriePete said, don't brake while swerving. Or cornering at all for that matter
Cost-wise, I got mine as a present, so I don't know, sorry! Advice-wise, all I can say is to check your shoulder before moving away EVERY TIME, even when you're not even on the pad. You may not think he's assessing you, but he is. Before the test, reverse park the bike, so that you can just ride straight out. But other than that, just ride according to the highway code and you can't go wrong. Part 2 is just like a car test, really.