JW's being cheap? Not my experience. They are a cross section of the population, especially those in the lower income range. They are not better or worse than non-JW's. The only point I can derive from my experience is that the ORG can not produce more generous human beings. After the indoctrination, those with a good heart remain so, and the stingy ones remain so too.
If they are cheap, it may be because the ORG already squeezed so much out of them, and there's little left to pay for the restaurant that may be above their affordability range.