IT Service Pricing Implementation

I would like to know whether anybody can assist me on the IT service pricing implementation i.e the charged-back of IT services used by other departments within the company? What is the market benchmark on IT service pricing? Any guideline to implement it?

Currently we provide IT services to all departments free of charge as all or most organisations do. However, we wish to implement IT service pricing as an inter-department charge-back for the IT services utilised. Therefore, we would like to hear from various company on their pricing strategy and implementation strategy for this.