A PR Agency will (should) price this based on value to the client.

You would be wise to do the same and avoid the hourly-rate blackhole.

Work with your client to determine the value of the deliverable to them, and price accordingly. Internally, you should now how much you want to earn per hour, and how many hours it will take you. This is for your information only, not the client's knowledge.

If you'd like to further discuss value-pricing for your service, I'm happy to help.

Drop me a line,
-Shaun