How do I price a service I’ve never offered or delivered?

I have a client that I’ve completed work for on my standard services. They’ve asked me to help with another project but the work is something that is not part of my offering but I’m willing to try. My services are fixed priced packages based on the number of hours (data from past projects) I know it will likely take to complete. I’ve never offered this service or product creation. How would I go about setting the price?