I don't think I've ever worked with a code base that had 100% coverage so I can't speak from experience per se. But my general feeling is that the cost increase

I don't think I've ever worked with a code base that had 100% coverage so I can't speak from experience per se. But my general feeling is that the cost increases exponentially and the return on investment diminishes exponentially with those last bits of coverage. Also, I don't think I need to tell anyone here but "100% branch coverage" still doesn't mean "100% branch combination coverage", which would quickly make your test code base grow to the size of the universe anyway.