Yesterday I went to see the backup team to see the ways we can reduce the cost of the backup ( recession time ...cost cutting ;)
I was told by backup team that oracle is taking lot of resources w.r.t tape ..thus they asked me to find out for each database what is the size of the backup ..
It is very difficult to get the size of the rman backup's directly but following SQL will help u to get the approx size in TB about the rman backup sets in tape . This is valid only for 9i ..It may work in 10G also but not checked and tested !!!
select sum(blocks*block_size)/1024/1024/1024/1024 from rc_backup_datafile ;
select sum(blocks*block_size)/1024/1024/1024/1024 from RC_BACKUP_REDOLOG ;
Above SQL's have to be run in sqlplus in RMAN catalog database ..
Hope this helps to determine the cost of tape's