Events Payment - quirky scenario
Check out what you can achieve by using the Batch Update by Profile process.

Create a Profile that has the Fee Amount field for Participants.

Go to Find Participants. Search for those that attended your event. Under Actions click on Batch Update by Profile.

You should then be able to enter the required amount for the top row, and use the 'photocopy' icon above the Fee Amount column to apply that to all Participants.

If you want to record the 'total cost' of the event you could also look at adding a Custom Field on to Events and use that to record the total cost of the event.