Paid Registration with Paypal I have visited related questions here but i need more specific answer.

I have hooked into the user registration form and i have added a custom submit handler for user registration before the user drupal core registration function.

I want to implement paid registration. I have visited Paypal IPN documentation but they said the require a return url.

Actually i am confused in the whole process. I want to know actual steps to implement payment api of paypal then if the user has paid the drupal core submit function should run otherwise i'll die the script.

Thanks
If this is an activity that you'll be repeating, like hosting events on a regular basis, I would step it up a notch and add CiviCRM to the Drupal installation. It will increase the complexity of your site by adding a new administrative area where you would manage the CRM components (at this point events and Payment Processors) but that additional complexity will simplify managing them. 

Out of the box it's set for managing events with PayPal; During setup you're prompted to go through a checklist. One of those items is Payment Processor. Set it to PayPal, follow the prompts and it just pretty much works.