How do you price web development jobs?

I have designed a few sites, some for companies, some for clubs etc but have never priced a job or supplied a bill for my services. I develop in .NET C# and use CSS for the presentation layer with either MS Access or MS SQL backend. My sites can range from 5 to 20 pages, most of them dynamic.

I am looking for a method or guideline of how i can price such jobs, opinions, resourcs on the interent etc.

Thanks

I also charge by hours. If you charge by the job, the job gets too much scope creep.

So my estimate is something like "This job with x, y, and z features will take 10 hours of Skill A at $999 per hour and 10 hours of Skill B at $50/hour."  That way when the client says "Could you just add this little thing", I say  "That'll take another hour of Skill A".

Clients understand something taking longer and getting it reflected in a bill a lot better than they understand "That'll be another $100".

Also makes it easier for me to contract out stuff that I'm not personally good at - like graphics.