New rear tire - 80$
Top end rebuild - 100$ (if you do it yourself)
Premix Oil - 50-100$ depending on how much you ride
Bottom end rebuild(maybe) - 200$ (if you do it yourself)
Chain and sprockets - 150$
Sparkplugs - 5-25$
This is a rough estimate in USD. It is also an estimate for an almost perfectly running bike, that has a perfect frame, and this estimate also assumes that you don't crash badly enough to bend, break, or crack anything. (This is highly unlikely).
Like the guy above said, It really can depend on how much you ride it, and whether you take care of it, and whether the previous owner took care of it.
To get a more realistic idea of your riding costs...add those numbers up...then add about 300-500 dollars in parts AND GEAR!!!!