Something you should consider is "perfect market" vs. "real world".

In a perfect market, outsourcing is the main way of taking advantage of economies of scale. You don't run your own national telecommunications network, you outsource it to the national network. You end up paying (cost - economies of scale + profit). The trick is, if you can reach those economies of scale with your datacenter, and you're a competent bunch, you end up paying (cost - economies of scale). So you can save money. There are obvious security and accountability advantages too.

That's the perfect market. In the real world, these folk charge far more than (cost - economies of scale + profit). They cater to inept organisations who couldn't collaboratively tie their shoes up without a contractor to show them how to do it. So you end up paying (cost_of_incompetents_doing_the_job - economies of scale + profit). The profit part of the equation is miniscule compared to the differences between "cost" and "cost of incompetents doing the job". If the home-grown data center would be big enough (I don't know that it would be, given the brief description in the post), and if it was competently-run, then you can save huge amounts of money by doing it in-house (again, aside from all other benefits).

This post has dealt exclusively with cost. Personally I would consider the other factors (security, accountability) to be the deciding factors, assuming that both options implemented services competently.