It's more of an issue of retaining quality people.
I don't have a clue what the hiring costs are for other endeavors, but I know it costs my former employer $250,000 to field one basic-trained rookie police officer. We called them "fours" when I was a trainer because that was the minimum acceptable score in any graded behavior ("eights" walked on water). A four could be counted on to drive a police car without wrecking it on most nights, could find his way to wherever he was sent within a reasonable time, could investigate simple crimes and was generally safe enough that we felt comfortable about him being out there on his own. Even then we seldom ever let them ride by themselves even after a year of field training and evaluation.
It takes 5 years to make a well-qualified police officer who is really effective at his job. That's the starting point for anybody who wants to move up in the organization and investigate real crimes, or supervise all the neophytes. A 5-year patrol officer is the average street cop, and he/she only accounts for half of the organization. What did it cost to produce this basic building block of police work? A couple of million maybe would be my rough estimate.
And those are the people that all the other organizations want to steal from us because it saves them so much money in hiring and training.
In the early 80's there were only two police agencies in the country that paid more than us (LAPD and Anchorage), and we were still losing 5-7 year experienced cops left and right. Some of it was working conditions (we were the murder capitol in 1981), but a lot of it was over pension. Several friends went to work for agencies like the DEA, Secret Service and Customs and all told me that they took a pay cut to do so. At the time our pension did not compete with what the feds and a lot of other agencies were providing.
I had a patrol partner in the 80's whose secret service application was stuck because of the Reagan federal hiring freeze. Every night I had to listen to how great the retirement was going to be if he ever got hired there. And at the time it was a joke that all our pension meant was that you could sleep in a flophouse hotel instead of under the bridge, and you could buy brand name cigarettes to go with your MD 20-20 wine. When I was a detective we used point to winos on the street corner and say, "look, there goes a retired detective."
You can spend the money to hire quality people and then fail to spend the money to retain them and still suffer as if you were hiring garbage at the front door. All the people with potential to be something other than a basic worker all have what it takes to get hired elsewhere, and many of them start looking at their future with different considerations after they've been there a few years. When they leave you wind up with decent patrol officers but incompetent supervision, investigations, management, etc. Before long the patrol function would just fail as well.
It's not that you can't hire people to do these jobs on the cheap. The problem is what you get when you do it on the cheap. Atlanta PD in the late 70's and early 80's was an underpaid department that self-destructed. New Orleans PD was the same thing in the 80's and 90's (and some would argue the 2000's as well).
Cities get the kind of police department that they pay for. You pay to hire quality and to retain it.
Just to prove that my former employer was a dream to work for, in a few short years they managed to turn the situation all around and make the pay some of the least-competitive in the country while improving the pension to something to hang around for. And that's when we started getting some problems in the lower levels of the organization because we weren't hiring the right folks. And the transition period was fun too - morale was so low that a common refrain was, "Screw it, it's not my house getting broken into. We'll get there when we get there."
There is no pleasure in having nothing to do; the fun is having lots to do and not doing it. - Andrew Jackson