Not that this is anything like the same league as the other bikes in this thread, but probably worth the exercise while we're on the topic.
Donor bike: $200 (part of a package deal), Snowy76.
Non-reed top end: $0, swap with FactHunt (where is Kurt?).
New piston: $50, Ebay.
Rebore: $90, local engine reconditioner.
Engine porting: $0, me (and lots of time on the phone to Lozza...).
IT250 electronic ignition: $100 (? - it was ages ago), Mad Max.
Pipe: $30 in bends, local muffler shop.
Plug cap, plug, fuel hose, etc: $20, local bike shop.
30mm RT1 carby: $10, part of a big pile of stuff I bought off Tim754 years ago.
Air-filter: $20, local car-parts shop.
New handlebars: $90, local bike shop.
New grips: $15, local bike shop.
New lever blades: $20, local bike shop.
Paints: $50, local hardware store.
Fuel tank: $5, part of a big pile of stuff I bought off Tim754 years ago.
Tank strap: $10, local saddler + stuff laying around.
Footpegs: $46, pit-bike shop.
Decent forks : $80, local wrecker.
New fork seals: $30, local bike shop.
Front rim: $0, came on a $200 YZ125E.
Front spokes: $35, Epay.
Front tyre: $20, local gun rider.
Rear rim: $0, my shed.
Rear tyre: $0, a favour from MBoddy.
Rear guard: $0, Adam S.
Front guard: $0, my shed.
Front number plate: $0, my shed.
Side number plates: $0, Adam S.
Numbers and back-grounds: $20, local sign-writer (materials only).
Oils etc: $50, local bike shop.
Call it $1000 and you'd be on the money.
I think that covers everything that I've spent money on, or would normally have expected to spent money on. Everything that I haven't mentioned either came on the bike (seat, shocks etc) or came out of my pile of parts that "might come in handy one day" (cables, chain, kill-switch, etc)...
My work also sponsored some bits and pieces (legitimately - the big boss insisted on a clean up, so a lot of stuff was saved from the bin), and use of tools like the bead blaster and the welder.
In hindsight, it was a good bike to start with - the botton end and gearbox are sweet and haven't needed to be touched, the frame is clean and straight, etc.
Here it is, ready to be loaded onto the ute to go to CD6 and then the Nationals.
And here's a photo that I think is cool but can't think of a better place to put...