Yeah, I would not finace a dirt bike uless you just dont have all the scratch. Best thing to do is put as much as you can down, and pay it off as quick as possible. Stealerships will nail you with like %18 interest, wich is bad. Dont get stuck in debt when your young, wait a year, put that years worth of savings down on the bike or, better yet, buy one that is a year old with lots of mods for less $$$. Lots of dealers re-sell thier soponsered race guys last years bike on thier used bike floor and will also finance them.
Also, if you dont plan to MX race, trail bikes hold thier value much better. Bikes like TTR's and XR's. Belive it or not, probably the best resale value dirt bikes out there are kids dirt bikes like PW 50s and XR 50's. Not that your going to get one of those, but the lesson is family bikes have great re-sale since guys look for them for thier wifes, or kids getting into riding.