"From a cost standpoint it makes sense." Only if you view education as a cost center. What's the ROI of spending on gifted education? I've not seen such studies; however, what if you could dramatically improve GDP, etc. based on supporting gifted individuals.

"How many students will use a gifted program?" Approximately 6 to 10% are gifted

"They will require dedicated teachers and staff, and their own curriculum." Not necessarily. They may (depending on levels of giftedness); however, differentiation and what not are also acceptable approaches in some cases.