Which engine? There were 4 engines available for 2004: 1.8T (turbo), 1.9L TDI (turbo deisel), 2.0L, 2.8L VR6. They all have slightly different schedules. But, that said, the engines are all interference engines, and will very likely sustain significant damage if the belt breaks. At 90K miles, it's probably a good idea to change it given the age of your vehicle (rubber will break down over time). And, yes, it is an expensive job. While you are getting the belt replaced, you might want to consider replacing the water pump. At 90K miles, your are getting close to needing a new one, and they will have everything disconnected already.
BTW, when VW says check, that's what they mean. And, the result of the "check" can be a recommendation to replace. Otherwise why check it in the first place. I mean, you are asking them to look at the belt, and use some judgement in assessing its condition. They start checking the belt at 60K miles. The recommendation is left up to the mechanic, but in this case, I'd go along with the recommendation.