I think it makes sense for policing to take cost into account. If you're rationing limited resources how else are you supposed to do the most good with what you've got? Of course, you have to take into account the wider impact of these decisions, for example if a certain crime just never gets investigated because it's expensive, people might move into it more. Other things being equal, I think that if there's a way to enforce the law which pays for itself that's a good thing as it leaves more money for the rest of the public sector. I think part of the issue in the example given isn't that the police are pursuing crimes which give a financial return, but more that the particular crime in the example the crime shouldn't be a crime in the first place. If, on the other hand, police were able to fund strict traffic policing through levying hefty fines upon people who drive dangerously, I'd be all for it.