The new business edition of custom search comes with a sliding scale price tag, the Google site details.
According to the site, Custom Search Business Edition is available in a number of plans:
Search less than 5,000 web pages: $100 per year
Search less than 50,000 web pages: $500 per year
Search less than 100,000 web pages: $850 per year
Search less than 300,000 web pages: $2250 per year