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Startup costs for running IM server for iPhone App?

Excuse me for not being technically versed (I am more of the creative type/businessman), nonetheless I am looking to develop an iPhone App that integrates instant messaging into the product for all my users. In other words, I am anticipating on having several thousand users using the App and engaging in IM conversations at a time (I am creating a community within my app for users to interact with each other.)

Does anyone have any sort of benchmark on how expensive it would be to run an IM server? Is Instant Messaging very server-intensive and if so how much am I looking to spend on average? Any recommendations on hosting solutions? MediaTemple?

I realize these are subjective questions, but I would like to know what I am up against in terms of cost.

Thank You,