Unfortunately, this situation places you in an awkward position. I have seen movements to IT outsourcing, Accounting outsourcing, and Engineering outsourcing. I know of success stories and I know of disaster stories.
Most of the disaster stories I can recall could have been predicted to be disastrous given the timing and adoption of outsourcing. Generally, outsourcing is sought at a time when financials are in a weekened state and short-term solutions dominate the thinking process. Adoption of an outsourcing strategy MUST also incorporate a change in how the company as a whole operates and conducts business. It is a business change and a process change, afterall.
Your company cannot continue to function "business-as-usual except outsourcing IT." It will have to change. Having your division as a pilot project for the entire company may prove to be difficult if this concept has not been addressed. Factors other than cost are also timing issues (how long to get or deliver the information), exchange of information (how, where, when, what), and required ownership (audits, reports, ownership).
I don't envy your position. I also think you have been given an assignment that requires a lot more involvement than those giving you the assignment realize.