step 1) figure out your hourly rate you'd like to be paid, a fair one step 2) estimate how long it's going to take you to finish the job step 3) multiply those two numbers.
#1 depends on where you live, creds, professionalism, reputation, competence... all the factors that would convince a client that the quality of your work and your services are worth X per hour.
More factors: Is this something you've done before? Do you know all the "gotchas" that a PayPal gateway entails? Do you have pre-rolled code that you can repurpose for this project? Is there a shopping cart involved, or just an IPN handler? Does the client have an SSL certificate?
LOL. rocknbil that reminds me of a buddy uses a well-known formula to figure out markup on his freelance work. It's the one for converting Celcius to Fahrenheit.
F = 9/5 (C+32)
It means his minimum price is $32 to do anything. (0°C is 32°F). A project that will cost $1500 in man-hours will be invoiced as $2732.
I should have mentioned in step one; that number includes your cost of living and expenses and insurance on your equipment and printer toner and the phone bills and snacks and so on. It's not the same as what you'd expect as an hourly wage working for an employer.