Totally depends on the required functionality and scope of the project. A single static webpage with an email link might be worth less than $100. Something with dynamic functionality, database integration, and customer specific business logic could be worth tons more.
Typically I charge $75 an hour for something challenging, but that can easily fall to $45-50 an hour for an interesting project or when I am getting hungry.
The bottom line is that anything is worth exactly as much as you can get the client to agree to pay you.