The most obvious answer I think

Salary is also good - but salary is for most people a fixed figure, so the concept of an average for a single person seems strange. If the topic was the average pay for e.g. grade school teachers age 40-49, or the average monthly salary cost in a small business (this seems likely if you want the accounting term), you would phrase it differently - like "gjennomsnittlig månedlig lønnskostnad".