Operating costs are highly dependent on the flying hours done rather than by the month. This is obvious for fuel, but also for many other aspects, such as crew costs. If your crews do 90 hours per month, how many crews you have to pay depends principally on how many hours you are flying. Optimising all this is what those people in suits going to the airline head office, and who get parodied by the flight crews (OCCASIONALLY with good cause !) do for a living.
For a good indication, look at a major airline annual report. Many of these are now published on line in the airline's "investors" section of their website. There you will see reported the total costs for the year, which you can divide by their total flying hours (also often in such reports) to give costs per hour. When you have looked at a few you will notice wide variations. Don't forget that a brand new A320 and a 15-year old A320 will have somewhat the same operating costs, despite one being worth maybe 10 times what the other is worth. There will just be more maintenance costs for the older aircraft. When these get too excessive, you sell it and buy a new one. That is a hugely simplistic statement, there's a lot of arithmetic to be done to work it all out.
What actually makes up the "costs", and how you allocate the share of the Chairman's Bentley and everything else, is something that every accountant and manager, in aviation and in general business, has a different view on. Many of us make a career out of it
Don't forget, once you have looked at all the costs, to look at revenue as well, to be sure you are making an adequate margin. Otherwise, what's the point ..... ? Now you're well on your way to Airline Management 1.01. Actually you are on your way to being an Aviation Consultant. That Airline Management university course tells you how to do all the calculations. The consultant knows what all the key values typically are off the top of their head.