How much should I ask?

Hi,
I would like to know how much I should ask for a project. Here what I will do:
- 7 html pages with few php scripts and 1 flash video player
- 5 php pages to manage users data and 1 flash video chat (1 to 1)
- 10 php pages to manage the website and anything else (administration part)
- online payment system

I've never been good to define the cost of a project.

I would be thankful to you.