First: I am not experienced with ARMA mission coding or with ARMA, all I know about the game is that it exists.
Even though I never coded anything for ARMA, I have some experience on debugging on languages that I never used (there are some languages that my only experience with them is fixing bugs on other people's codes).
It is hard to give a price or delivery date for this project because I don't know how big is the project, so I got an average of what I expect to face.