You know, if you just break it down into parts its pretty easy to see what you can expect to spend and at the same time maybe give you an idea of how far you want to go. One thing for sure, if you give it some thought first you will avoid false starts, which is a key to any accomplishment,
You have to have a bike to start with. If you plan to build don't buy a show bike to start with. Figure between $300 and $1,500 for your base bike, depending on where you live.
Tires will cost you $100 each
Rebuilding wheels cost $300 each (bearings, spokes, cleaning, PC or chroming, lace and true, tubes, strips ...)
Getting your carbs right is going to cost you $100
and then another $100 for plugs, points, condensors, oil and filters.
Chain and sprockets will run $100~150
New Brakes, $100
Seat, tank, bodywork, $100~$1,000
Paint, chrome, polishing, $100~ ....
Controls and fluids $100
These are just start up costs, so to speak. You notice I didn't include any money for engine repair or rebuild, not a dime for new exhaust, nothing about coils or wires, nothing about lights, machine work, or handle bars. Oh, and then there are the tools and supplies - a gallon of acetone cost $20, about the same price as a very cheap impact driver - both of which you will need.