Keep in mind that I have never cruised so I don''t KNOW, I am only offering an uninformed opinion.
I think rather than say the cost of cruising "depends" or "varies" it should just be said "it costs what you are willing to spend".
I have a wife and 2 small children, house payment, car payment and all the other expenses. I estimate my total monthly expenses at $1600 (no more than $1800 since that is all I earn) I can''t imagine that it would cost more for me to live aboard than it does on shore. I really believe (and I am sure you will all tell me I am wrong) that I could live comfortably but humbly on $500-$600 a month while cruising.