The base price is calculated by multiplying your hourly rate by your time estimate. We then add some extra time for contingency in case of unexpected problems. Finally we round the base price, to make it look nicer.
Next we look at how important the job and client are to you and adjust the rate accordingly.
This gives you a base rate - and an ideal rate. You should quote the ideal rate - knowing that you can drop to the base rate and still get a fair deal. Of course, you should always stick to the ideal rate if you can!
Read more on how to charge more as a WordPress freelancer.