The cost has to be looked at relative to the actual benefits received. A cost/benefit analysis.
Having an overly brutal police forces costs society much more than the above analysis and has no added benefit. Single incidents can cause large amounts of social strife and increase distrust of policing in general.
Further, any such analysis of our overly costly prosecution of our overly punitive legal code will tell you that how we treat non-violent offenders especially doesn't really benefit society.
Models that focus on rehabilitation of such offenders do a better job insuring peace in society and cost less relatively.
That could very well be true...however, the nature of lawsuits in our country (for anything) have trended toward being more costly, and legally speaking, a person can sue for anything.
For instance, if I try to rob my neighbors house, and the cops catch me, and tackle me, and I claim they hurt my shoulder, there's nothing legally stopping me from filing a large suit against them, even if I didn't win, it still costs taxpayer money for both the court's time, as well as the legal fees of the lawyers for the officer.
That gets exacerbated especially in cases where the circumstances are very public, and certain attorneys are looking to make a name for themselves.
I guess the onus would be on the camp suggesting we have a "overly-brutal" police force to use the phrase you mentioned.
There's no doubt there are some problems, and those problems, per capita, are more prevalent here than in other developed nations...but the flip side of that coin is that police in those other countries aren't having to respond to the same types of calls as frequently as they have to here.
For instance, I'd be highly shocked if Norwegian police forces had to respond to as many calls pertaining to things like gang-related activities, "armed robbery in progress", etc... as police do here.
I've long maintained that it'd be an interesting social experiment (although, I don't see it ever happening) to take a police squad from one of these other countries, pay to have them come over here...and let them try our there approach and see if
A) their methods are effective in a US-style environment
B) if they make their approach harsher over a period of time after being exposed to it.
IE:
Let's bring over the London Metropolitan Police Dept... we'll completely stay out of their way, make sure they have any tools at their disposal that they'd like to have, and give them complete autonomy with regards to how they want to do business...and give them the city of St Louis or Detroit...and we'll see how they do for a few months.