To add a registration fee to you event is a simple two-step process
1. Go to the settings for the distance you want to add the registration fee for, and make sure that "Registration fee" is toggled in the section Additional.
2. A new section, Price, will appear when this is turned on. In the Price section you can now add the registration fee. You can add more than one price, if the fee will increase closer to the event.
Save and the registration fee will be added to the distance. You can add different or same pricing for your different distances.