Google Site Search has the following options available, depending on your estimated query traffic on your website:
Google Site Search pricing
| Search query limit (annual) | Pricing (annual) |
| 20,000 | $100 |
| 50,000 | $250 |
| 150,000 | $750 |
| 500,000 | $2,000 |
| 500,000+ | Contact sales |
You can sign up at http://www.google.com/sitesearch and pay online using Google Checkout.