Is there a way to display a calculated field in a
custom report, for example I want to display the value of (event X / event Y) over time?
average event value,
total events, etc.)
No, I don't think that Google Analytics offers calculated fields yet. You'll need to extract your data via the reporting API, then manipulate it using Excel or a database.
If you don't want to write code to use the API directly, there are tools available that you can use to extract your data: For example, Excellent Analytics, is a free tool for downloading Analytics data into Excel.
We solve this by using ZohoReports which has very strong capabilities for doing aggregated calculations that can be charted / filtered / and grouped along different time periods without doing any additional work. To get the data into zoho we use a mysql database and populate it using itduzzit.com - we've been running this model for almost a year - lightning fast and completed automated. you can also embed the charts and pivot tables in a google site with different permissions depending on the audience.