Hi Mate. I'm guessing it will be a silly amount at the stealer.
Kits I know you can pick up quite cheap on the net, have a browse - but my VW mechanic friend told me that apparently if you get the genuine kit from the stealer, although around £100 ish (he hasnt confirmed so may be cheaper), you still get the parts warranty
which is better than a cheapy one cos obviously you don't want your new belt to snap. Get these parts then you can get a local garage to fit for a much much reduced labour price
Also, while you're at it, get the water pump done too, and put a metal impeller version on as the plastic ones are susceptible to failure.
Linky for water pump
http://www.water-pumps.org/Audi-A3-1-8-waterpumps.html
Have a look on that site, you'll need your engine code
Hope that helps