You need to know two things:
- what is your time worth per hour?
- how long will it take you to do the job?
The first is something you need to work out for yourself - how much do you want to earn to make it worth an hour of your time? Remember you need to pay tax. How much is reasonable for your skills and local market? For example, we might all like to earn £200/hour for Wordpress development, but that's not realistic...
The second is simply an estimate. You should know how long it will take you to do the work (if you don't, you shouldn't be doing it). This will depend on exactly what you're setting up and how much customisation is required. Multiply by the hourly rate you worked out.
Once you have a final figure, you can offer a quote based on that, adding any hosting or support retainer as required. I assume you won't be charging VAT so don't worry about that (but make clear it's not charged).
A *basic* guide for how much to charge for a freelance hourly rate is to double your salaried hourly rate (assuming you work in the same field and at the same level as you intend to freelance). That doesn't take into account local market though, so it's only a starting point. I'd say £20/hour would be about the ballpark for Wordpress setup - it's not hard. I'm sure you'd get away with more though as you've already been approached!
Does that help?