That's a bummer to hear about a timing-belt failure; this type of event is the reason most Audi owners are pretty cautious about monitoring the time/mileage that their timing-belts are in-service (3.0L V6's timing-belts are usually changed at 75K-mile/7-year intervals). Please be aware that the Audi engines are all "interference" designs, so under normal operating conditions, the valves and pistons alternately occupy the same space within the cylinders. A timing-belt failure means that lots of expensive parts will catastrophically meet one another at high velocities. I'm unaware of any means of beginning to accurately assess the carnage short of pulling both heads from the 3.0L V6 engine. In addition to the valve-train and the pistons, you'll need to determine if the cylinders have sustained any damage.
Depending upon the extent of the damage, you may need to look into procuring either a short-block or a long-block replacement engine. The "short block" includes the engine block, pistons, rings, pins, connecting rods, crankshaft and main/rod bearings. The "long block" adds the cylinder heads and all of the attendant valvetrain components (valves, camshafts, variable-cam sub-systems, etc.). A quick Google search turned-up a couple of Audi engine rebuild specialists with available stock of B6-platform A4 3.0L V6's at ~$3500 for a short-block and $5500 for a long-block replacement engine (of course, shipping costs will add to the bottom-line....).
I wish you good luck in restoring your Audi to operational status!
* 2003 A4 3.0L Quattro (6-speed manual)
* sport package
* 18"x8" Moda MD5
* Sumitomo HTR+ (235/40ZR18)