If "copious comments are great" is true -- does that mean we should write copious comments? I'm talking about opportunity cost here -- the time spent writing c

If "copious comments are great" is true -- does that mean we should write copious comments? I'm talking about opportunity cost here -- the time spent writing comments is time not spent on the code, design, test, etc. So for me the question becomes what I should do to provide the max benefit for the least cost -- should I write comments, improve the design, fix up some code to eliminate special cases, test for corner cases? Since it's an inherent trade-off, I don't see this as being an easy question to answer. Probably, there are multiple adequate answers.