I built up a Nashbar mountain bike frame with one of their carbon forks & disc brake sets. It's a good frame for the price, $80 after discount. In hind sight, I'd use a regular steel or suspension fork & V-brakes. The only real complaint I have is that my size 12 feet can hit the chainstays.

Maybe your (and my) project don't make perfect economic sense. The bookkeepers aren't accounting for the fun factor. I learned a lot & ended up with a good bike.