How would you charge for public relations services?

We received an email from a new public relations practitioner asking how he should charge for his services. I’d like to share his email and my response.

The question:

I’m an up and coming media/PR consultant.  I was in the media industry for 13 years and now I’m looking to develop my own consulting business.  …

Recently, I acquired my first client.  I’ll be paid on a per project basis. The client wants me to work on a social networking website campaign I suggested.  Basically, I’ll be putting together this small business’ Facebook and Twitter pages.  I’m trying to figure out how much I can charge this business.  Should I go with a per hour rate?  If so, how much?  Or a flat rate? If so, how much?  I’m also trying to determine how long it will take to build traffic and interest to these social networking pages.  I’m guessing it’ll take 4 to 5 months for any substantial growth.  This company is looking at this campaign as a new way to attract interest to its website/store.
I’d appreciate any thoughts as to how much I can charge.

My answer:

What you charge depends on the overhead you must carry (your needs), the value to the client (what you should charge), the budget of the client (what you may only be able to charge.) Bottom line, I’d start by asking for a fee equal to what I think the project will be worth the the client based on anticipated results. Then, if they cannot afford this, you can decide whether to negotiate an acceptable fee.

Your thoughts:

How would you answer this question? What’s the right way to charge for public relations services?