I can say from experience that there is usually a WORLD of difference between a concept and a street car.
On the concept, they don't have to worry about being road legal, so they can make a one-off frame out of tube metal, they have their fab shop do up a few panels, etc, etc.
There is a MASSIVE expense in taking a concept and making it a reality. basically you're doing 90% of the work taking a concept to fruition - alot more than most would think, I imagine.