Average annual maintenance cost will depend on several things:
1) if you do most of the work yourself, much less
2) if you want to keep the car in nearly perfect condition, much more
3) how is your luck?
I am spending quite a bit even though I do a significant portion of the work myself....but my standard is high as I want the car to drive, ride, and look almost new.
If you don't want perfection, can do some work yourself, and have a bit of luck, a grand a year should be the ballpark. If you are more like me, think two to three thousand a year (on average). Still, even at three k, thats similar to buying a relatively inexpensive new car and in my state I save at least $500 a year on property taxes (cars over 15 years old pay only $15 a year in tax here!).
'85 300CD @ 150k miles.