That ignores higher order effects--how theft changes primary behavior. In your example, fear of pick pocketing may reduce the degree to which people carry around and spend money with vendors. Your rapacity index makes little sense: by your logic, corruption has a low rapacity index, since the state has a lot of money compared to the amount of the transaction. And given the deterrent function of sentencing, the more relevant figure is the aggregate effect of a particular class of theft, not an individual instance of theft.