I guess this question has been asked a lot around. I know Rails can scale because I have worked on it and it's awesome. And there is  not much doubt about that as far as PHP frameworks are concerned.
I don't want to know which frameworks are better.
How much is difference in cost of scaling Rails vs other frameworks (PHP, Python) assuming a large app with 1 million visits per month?
This is something I get asked a lot. I can explain to people that "Rails does scale pretty well" but in the long run, what are the economics?
If somebody can provide some metrics, that'd be great.
IMHO I don't think the cost of scaling is going to be any different between those three because none of them have "scalability batteries" included.  I just don't see any huge architectural differences between those three choices that would cause a significant difference in scaling.
In other words, your application architecture is going to dominate how the application scales regardless of which of the three languages.
If you need memory caching you're going to at least use memcached (or something similar which will interface with all three languages).  Maybe you help your scalability using nginx to serve directly from memcache, but that's obviously not going to change the performance of php/perl/python/ruby.
If you use MySQL or Postgresql you're still going to have to design your database correctly for scaling regardless of your app language, and any tool you use to start clustering / mirroring is going to be outside of your app.
I think in terms of memory usage Python (with mod_wsgi daemon mode) and Ruby (enterprise ruby with passenger/mod_rack) have pretty decent footprints at least comparable to PHP under fcgi and probably better than PHP under mod_php (i.e. apache MPM prefork + php in all the apache processes sucks a lot of memory).
Where this question might be interesting is trying to compare those 3 languages vs. something like Erlang where you (supposedly) have cheap built-in scalability automatically in all Erlang processes, but even then you'll have a RDBMS database bottleneck unless your app nicely fits into one of the Erlang database ways of doing things, e.g. couchdb.
Cost of scaling Rails vs cost of scaling PHP vs Python frameworks