The maintenance isn't calculated on aircraft-to-aicraft basis. The script uses the average aircraft age and aircraft value, among other variables. When you recieve your new aircraft, you might see that your maintenance fees fall, but sometimes, it suddenly jumps several times higher.
Don't expect maintenance to be consistent short term, but you will see general trends when comparing many different values.
Like Agent O. said, you can take note of your current maintenance fee and do the same when you get the new aircraft. This will give you an idea of how much the new aircraft costs the first while.
