How much should I (working as a freelancer) charge for creating a custom WordPress plugin?
The short version:
Figure out how much it costs you to eat, go to movies, pay utilities, pay taxes, etc., each month. Divide that by the number of working hours in a month. Congratulations, that's your minimum hourly rate.
Get the specs in writing, as detailed as possible. Then figure out how long it will take you to develop, test, and implement the plugin as per the specs. Multiply the number of hours the project will take you times the money you need to live. That's how much you need to charge for your project. At minimum.
Get the specs in writing, and don't deviate from them unless your time suddenly is no longer valuable to you or you decide you don't need to eat. Put in writing that deviations from the specs (feature creep) will cost $X per hour additional, where $X is your hourly rate.
Keep in mind that if you're freelancing for a living, you will not have work 100% of the time, so your hourly rate will need to encompass the amount of time it takes you to find more work and close a sale. So if you're doing this for a living, go back to the first paragraph and add in the average amount of time it will take you to find work and close a contract, then add that to your hourly rate. That's why many freelancers and contractors will charge less for regular clients or for retainers; they can afford to because they don't have to spend extra time finding the work and negotiating the fees.
Also note that there are other people who will charge less than you. If you enjoy your current standard of living, do not compete on price, compete on quality and reputation. So let other people charge less, and make sure your work is good and that you meet deadlines.