A factory service manual will be essential when you get your bike.
Meanwhile, a good reference for you would be Eric Gorr's Motocross and Performance Handbook. Page 29 lists some average maintenance intervals for many components. Ultimately those costs are going to be determined by on how often you ride, conditions that you ride under, and the condition the bike started out to begin with. Since you are looking for a used bike, you will want to budget higher, rather than lower.
Not included on the maintenance interval chart are things like gas/oil/tires; entrance fees if you end up at an mx park, or ride in trail areas which require a fee, or the cost of getting to those places. (You'll want to offer to contribute to your buddies' parent's gas if someone takes you someplace to ride. Just good etiquette.)
Definitely follow this book up with the service manual for the bike you purchase. You can't just wing it. You'll be doing lots of tweaking. Good job on researching the total costs to your sport before just plunging in!! :cool: