I have no idea, i guess it's different for everyone.
Me for example, I have about $300.00 total in one of my bikes, and about $350.00 in the other.
this is including the cost of the bikes themselves, along with the engine kits, new tires, custom paint, etc.
I do all of my work myself, I make my own parts when i can, and I do my own painting.
I built my first bike in 2008, and it's still running strong.(49 c.c. 2 stroke slant head bought from bgf).
the engine has never been rebuilt and it runs as strong now as it did when i first built it. the clutch pads are still the originals too.
I built my second bike in 2010, and it too is still going strong. (66 c.c. 2 stroke slant head bought from bgf).
I did have to replace the clutch pads and the intake manifold gasket on that one, but everything else is original.
of corse on both of my bikes, i replaced all the nuts and bolts with better ones, replaced the exhaust gaskets, put on better pipes, tuned the carbs, run high flow air filters, ngk spark plugs, accell plug wires, and blue loc-tite on every nut & bolt possible.
internally tho, both of my engines are stock and original.