Paid Registration with Paypal
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.