I am assuming you are not the councilor.
Civil matter you have no standing. The councilor does. Maybe the hotel bar does. I can’t think of anyone else who does.
Criminal matter If a crime was committed in DC, the DC prosecutor is under no obligation to prosecute it. As a political matter, if you were a citizen of DC you could complain and try to get the prosecutor voted out next election.
Political matter As a citizen of your city, it is a political matter: You call your councilman* or any city councilman really and ask them to begin impeachment or whatever similar action you have there. Then next election to try to get the mayor voted out. If you are unsatisfied with the council, try to get them voted out also.
The cost of investigation is a cost of running a government. Someone could have chosen not to spend seven grand investigating a hundred bucks, but they did it.
*Note this is called petitioning for redress which is the right that the current SCOTUS nominee could not recall off the top of her head.