Maybe you should see how long a piece of string is,hey caps?
Seriously,as hotdog stated up to $4000 for a really good one.
Maybe $600 to $800 for an average one.I picked one up off a wrecker in Vic for $1000 back in 2006.
It had,unfortunately been stripped of all the electrics,blinkers,had a shitty pipe off another bike,and was pretty much a blue YZ,with a front non-working light.And not as quick as the YZ.I felt a bit pissed that it was missing so much gear.But,my fault for not asking more questions on its condition.
After market parts have seemed easier enough to obtain .ie;plastics,DG pipe,even the local Yamaha team,have been good in locating parts.
I enjoy riding my bike,its got loads of grunt and,once you get used to 'em they will pretty much go wherever you point them.Although I've seen some negative comments on the forum from guys who hate the YZ490 for handling reasons,I've only had one get off,and that was my fault.
It seems though,you're pretty happy with what you see.I reckon if you're buying a complete bike that needs finishing touches and some minor engine work,ask the seller what he reckons it's worth.Compare his price to what you are willing to pay for it,and in there somewhere you will arrive at a decent price thats fair for both of you.Hope this helps.
Cheers,Mark.