On my own car (Peugeot 806 TD) I have it changed at about every 25, 000 miles or so and last time it cost about �140, the old belt was showing signs of wear even at this mileage. The handbook specifies belt changes every 50.000 miles which I think is a bit excessive.
If the belt snaps an exchange engine (if you can find one)will cost you thousands plus all the work and expense of getting it fitted, and, in my instance, would far exceed the value of the car itself.
The price of a belt change isn't a lot to pay for peace of mind. If there is any doubt whatsoever about the age or condition of the belt on your VW get it changed, and the sooner the better.