hosting costs for rails applications compared with php

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http://serverfault.com – I asked this question on stackoverflow but it got closed for being of topic. Maybe it belongs here instead ? I know it is more expensive to host rails applications. What I should like to know is how much more expensive it will be. I am a noob then it comes to hosting but lets say you have an identical PHP (framework) application and Rails application and both use best practice for database setup, caching and so on. How much more resources do you think a large site like for example stackoverflow should demand from a rails app compered with the same application written in PHP? On a 512MB VPS (HowTos)