I know the part is only about $25. I have a 2000 VW Golf Gti with the 1.8t Engine. My car takes forever to warm up so I suspect it's about time to change the thermostat considering i'm at 180,000Kms.
On any car you're going to be paying a lot more for the labor than for the part to replace a thermostat. I've never done it on a VW but I'd guess maybe 2 - 3 hours. This means over $100 for labor. Possibly well over $100. That is of course if you're paying someone else to do it. If you're up to it go for it. Also, replace the gaskets you break the seals on (180000km!)
Not an easy one on the vw/audi 1.8 turbo. I'd say 3 hours if you've never donw it before. Most of the 1.8T's need the intake manifold to be removed to perform this.
Another thing on the 1.8T's: Make sure you replace the timing belt every 65K miles, regardless of what your manual tells you. I've replaced several 1.8T engines for customers due to timing belt, tensioner and water pump failure.
