To make such a calculation, you start by making a list of cost factors and find out the weekly costs for each of them:

- costs for your networking provider

- the hardware (renting price, or purchasing price divided by deprecation period, broken down to a price/week), including backup systems, backup storage systems etc.

- costs for any software required to run the system (per week, like above) 

- costs for the personal running and maintaining the system, the software and the hardware

- costs for the rooms where the servers are placed

- costs for the electricity, backup tapes etc.

- maybe some things I forgot to mention.

Now you need to analyse how each of these cost factors would reduce in case your company would - hypothetically - not offer the service any more (or "outsource" the service to a 3rd party).

The difference is the "total costs of ownership"/week of the service. Divide that by 750 millions, and you have the costs per API request.