Sign up for multiple events, get a discount
Since you are on Drupal, you can create a webform. (You'll need the webform_civicrm module)
Enable 5 event options. Configure each event to be one of your events. Enable the participant fee field and set the price to $100.
Create a conditional 
If event 1 and event 2 and event 3 and 4 and 5 are selected then price fields are set to 90.
Enable a contribution form for online payment.