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.
For anyone that stumbles upon this question, Google Analytics now offers Calculated Metrics.