I want to use CQG API, for developing a strategy in c#.
Have basic programming experience.

Is there anybody who has used it here?
Is there any additional cost for using the API, apart from paying for CQG IC?

Anything else i should know of?

The documentation seems to be for excel macros? Would have really helped if it had some documentation for using it with .Net too

Additional cost is 245 for trading+realtime data (excludes any exchange fees) and historical data packages start from $100 a month

Documentation is for general API interface, not for excel. CQG’s API is a COM interface. One can write to it from any environment that works with COM - the list includes and not limited to C#, VB.net, C++ VBA.
CQG provide numerous samples in each of those languages. Sample source codes cover entire functionality of API. Any additional logic needs to be implemented by user
We also have a list of certified partners who provide development assistance.