There are two main adages applied to bike building. Firstly, work out what you think it will cost. Then double it. Most people add up the cost of fixing the frame, wheels, engine, etc, and completely fail to take account of cables, fuel and lines, controls, taps, paint, rubbers, nuts and bolts, and all the countless fiddly bits and pieces that are only a fiver here and twenty quid there, but boy do they all mount up.
Secondly, when you think you're getting there, add twice as much time again as you've already spent. At least. .. Putting the main bits together is easy -the forks bolt on, the motor bolts in, the wheels might need spacers and sleeves, but mostly such things become immediately apparent. What takes the time…