Well, good is highly subjective but there are sources of information.
I guess one of the problems is which identity and where was it applied? Here's a couple of thoughts that might get you closer to metrics.
So for enterprise you might try to look at Larry Ponemon's Cost of a Data Breach and attribute what amount were caused by theft of identity:
For consumer, Symantec Lifelock seems to have commissioned the best/longest running study into this:
Experian has a study but no info on sample size:
Lastly, Equinfax has given us the biggest real world study by a practitioner*...
* sorry, cheap shot I know, but I couldn't resist.