httpwebwitch - 2:22 pm on Dec 17, 2010 (gmt 0)
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.
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?