That's true, but you might be missing the productivity gains from using a framework like Ruby/Rails -- you can throw a lot of hardware at a problem for less than a single extra developer's salary.
(won't make my argument about other, less resource-intensive languages being productive...that's just asking for trouble here).
Of course, productivity depends far more on the quality of the programmers and coding practices of the team than the language. I'd take a team of kick ass php guys any time over a group of inexperienced/douchy rails guys, and I hate php with a passion. Ceteris paribus I'd go with Ruby.
(Come to think of it, it kind of reminds of the martial arts comparison mantra: "It's the quality of the individual practitioner that counts far more than his chosen style")