For my first dozen or so of my SOHC CB750's (of various vantage) I usually spent about $150 to $700 on each of them and roughly sold them for around $1200 or more. But I was working at/ managing a bike shop and did the work myself, (mechanical and paint, etc) usually making them into mild café bikes.
For my current one I know I have more than $5000 invested; last time I calculated It seems it was around $7000 or $8000, but that just seems like too much now as I sit here typing it! It has extensive machine work and lots of expensive parts... Plus it is a '69 sandcast hybrid. The scary thing is that I am not yet finished!!!
Saying all that these bikes often can be purchase in running condition with no serious problems for $500 - $1000, depending on how good they look, and They often will need another $400 to $600 of work to get them running perfect -- for carb kits, battery,
tires, chain & sprockets, some tune-up stuff, and maybe some cables. Of course for a $1000 bike I would expect that all/most of that stuff would already be sorted out, and a $1200 - $1500 bike should 100% and not need anything.
In my book you only pay more than that for specials (i.e. café, chopper, really cool paint job, highly modified, sandcast, touring, etc), quality restorations, or especially nice and/or original specimens. Unless of course you are a freaking madman like me and are trying to build something special, then you'd better be ready fo a 2nd mortgage!
Hope that helps!!
Peace and grease
-fang