Is there much weight to be saved, by fitting a more specialized light weight crank as opposed to an off the shelf standard three sprocket crank, lets say from a 200 dollar mountain bike for example?
Weight saved with lighter tires and tubes will have a bigger impact on performance than weight saved anywhere else. That said, I think shifting and braking performance will have more impact on your riding than shaving a couple ounces.
Yup, also the wheelset will make a huge difference (well, the rim) concerning the weight. This will be most interesting in sprints, though, as the important issue is the moment of inertia: it takes more energy to accelerate a 2kg weight 60cm away from the axis than a 2kg weight (almost) at the axis.
A more expensive crankset will shift better than the low end stuff on cheaper bikes, so I'd say it is money well spent. Shifters as well as dérailleurs will also have quite some influence, though I'd guess the higher you go, the less pronounced the difference will be (it is basically less weight and better finish at some point, I'd say)