The high cost of bug-free FOSS is the time the developers can volunteer to develop it, which takes away from time spent on additional features. The developer's time is a limited resource that has to be spent where it is best utilized, and making the software bug-free might not be the right place.
Yes, good point. In both cases the limit is time. How do you think one should decide on what qualifies as best utilized? What would count as additional features? Does documentation count as a feature in software?