I think paid search should have a similar metric. Is the account growing because you are getting more efficient or are you just adding more keywords? New keywords are good but what money are you leaving on the table due to less than optimal use of current keywords because you’re too focused on constantly adding more? On the other hand, are you adding new keywords so infrequently that the potential for growth is limited?
Download two months of your account and do conditional formatting to highlight duplicates, then filter the keywords to see total revenue from new keywords month over month compared to same keywords.