The nice thing about building a "custom" bike from scratch is that you can build it up like you want, as you think you want.

It is, however, easy to get nickel and dimed to death.

$100 to $200 for your frame.
A few bucks for new track ends. A few bucks for installing them. A few more for repainting the frame (after massacring the rear end).
Wheels, tires, tubes, stem, bars, cranks, new fork ... it all adds up. If you aren't careful, your $100 bike has just cost you $500.