Do some shopping around. You can find donor bikes cheap--my riding buddy just picked up a 78 CX500 cafe project for 200.00.
Here is a rundown of the costs for my 76 CB750 Cafe--all of the parts are not yet installed--but it will give you an idea.
Also--check SOHC.net--one of the best CB forums out there for ideas, advice, etc..
bike--2,000
clubman bars/dunstall fibergalss seat pan/ rear LED light/(4) low profile signals/new grips/bar end mirrors--400.00
seat foam/custom cover with snaps--75.00
paint/fab work--myself
so, there is a rough idea for a 750.
Get the bike running right first before you do any mods--that way you have a baseline if problems develop.
I still have to install the dunstall seat/rear sognals/tailight--but here is mine:
