Cheapest way (platform/language) to implement a RESTful web API for an iPhone app?

[Nisse]

I am developing an iPhone app and would like to create some sort of RESTful API so different users of the app can share information/data. To create a community of sorts.Say my app is some sort of game, and I want the user to be able to post their highscore on a global leaderboard as well as maintain a list of friends and see their scores. My app is nothing like this but it shows the kind of collective information access I need to implement.The way I could implement this is to set up a PHP and MySQL server and have a php script that interacts with the database and mediates the requests between the DB and each user on the iPhone, by taking a GET request and returning a JSON string.Is this a good way to do it? Seems to me like using PHP is a slow way to implement this as opposed to say a compiled language. I could be very wrong though. I am trying to keep my hosting bills down because I plan to release the app for free. I do recognise that an implementation that performs better in terms of CPU cycles and RAM usage (e.g. something compiled written in say C#?) might require more expensive hosting solutions than say a LAMP server so might actually end up being more expensive in terms of $/request.I also want my implementation to be scalable in the rare case that a lot of people start using the app. Does the usage volume shift the performance/$ ratio towards a different implementation? I.e. if I have 1k request/day it might be cheaper to use PHP+MySQL, but 1M requests/day might make using something else cheaper?To summarise, how would you implement a (fairly simple) remote database that would be accessed remotely using HTTP(S) in order to minimise hosting bills? What kind of hosting solution and what kind of platform/language?UPDATE: per Karl's suggestion I tried: Ruby (language) + Sinatra (framework) + Heroku (app hosting) + Amazon S3 (static file hosting). To anyone reading this who might have the same dilemma I had, this setup is amazing: effortlessly scalable (to "infinity"), affordable, easy to use. Thanks Karl!Can't comment on DB specifics yet because I haven't implemented that yet although for my simple query requirements, CouchDB and MongoDB seem like good choices and they are integrated with Heroku.