The top benefits for me are modules. The template modules along with the CGI modules (and all the other various ones for databases, servers, etc.) are a huge pay off. That's not really a specific answer, though (sorry).
The "break even point" is hard to estimate. It will be higher for you than it is me due your larger knowledge of PHP and lesser knowledge of Perl. A site for a small business might be better off written in PHP because of it's availability and ease-of-use. The gaming site that you wish to someday create might be better off written in Perl because it is easier to divide the code and faster to write because the hard parts are written for you (see: modules). (I do not want to be specific here due to possible counter-arguments that will be pointlessly accumulated in your node.)
And you didn't even know bears could type.