This post is pretty generic in nature, but i would like to discuss the various options available to do cost saving on mainframe.
Some that come to my mind are:
-Using the cheap media to store backups 
-Make use of temporary datasets in jobs whereever possible rather than saving to disk.
-applying code tuning to get better performance.
- getting rid of licenced products and convert them to code in other products. for eg eztrieve conversion to cobol.
- Getting rid of reports which are not used any more.
- Removing redundant jobs and so on.