The way it was put to me when I looked into it was that they have urea “packs” that have to be replaced periodically, and good luck finding anyone who can do it other than the dealer. So, it’s expensive.
It’s not a “pack”, it’s a bottle of liquid. “Diesel exhaust fluid” is the generic term, Adblue is a trademarked name you’ll hear. The tank for DEF should be topped off during routine maintenance.
You need a diagnostic computer hooked up to the car to check levels, apparently - there is no dipstick on the (rather large) DEF tank. So for most drivers this is not a DIY operation like checking your oil level.
Here’s a video of a Passat being filled.
http://youtu.be/1h9hWlvg6sM
They talk about using VCDS, a DIY diagnostic software and cable setup, to check level. If you’re a serious DIY mechanic these days, you have to have it to work on modern cars. I’ve had the setup for around 10 years now.