Storage expanse are not to be overlooked. That a federal agency may be compiling and storing a lot of info doesn't make it any less costly for the local PDs to do so.
More importantly, most of the NSA type storage is all automated. With body cams, there is going to be a lot of manual storage and transfers to do. Sure, it becomes part of the cop's job. He finishes his patrol, fills out paperwork, downloads his body cam data, tags it appropriately, and goes on his way. I don't whether that is a 5 minute part of his job or a 35 minute part of his job. But it probably isn't 0 minutes until someone improves body cams and the mass storage devices to automate the process.
Then we add the costs of secure backup, offsite of course so data isn't lost to a fire, personal to run the system, and your typical local department is probably looking at an on-going 10% hit to their budget. That isn't overwhelming, but it isn't insignificant.
I suspect that the presence of cameras will prevent bad conduct on both sides in many cases. And in some cases, will provide evidence to either clear or convict the cop or suspect. But too many in our society will not understand that what a body cam doesn't show may still be very important. Too many don't understand that a body cam won't give them a movie-like view of real life situation.
Cost analysis below:
Depending on how long body cam videos need to be stored, we are talking about a lot of storage. According to this calculator, 1 hour of 1280x720 video at 15 frames a second (pretty low quality), encoded with H.264 (a newish and pretty decent standard) will require 13 GB of storage. That works out to 312 GB per 24 hours of officer street time. How many officers on the street per day?
According to this site most cities have between 10 and 40 officers per 10,000 population. Figure a nice median of 20. Ignoring both overtime and time not on the street, that works out to about 5 officers on the street, 24/7, for each 10,000 in population or the equivalent of 25 officers 24/7 for a modest sized town of 50,000 persons. That is about about 7.8 TerraBytes per day, every day. How long should video be stored before it can be deleted? 30 days? 90 days? A year? At 30 days, you're talking about 234 TerraBytes of storage needed or about 78, 3-TB hard drives. Retain it for a year, and you're up to 2.8 PetaBytes, or a little shy of 1000, 3 TB hard drives just for the 1st copy of the data.
A 5 bay disk drive based storage unit is about $1,000 on amazon, minus the disks. For 5 disks, toss in another $1,000. So $2,000 15 TB of storage. You'll need 390 of these for each 30 days you want to retain video so about $780k in hardware, per month of storage, or about $10 Million in hardware for one year's worth of storage. Double that for doing back up. We are ignoring electrical costs which for large storage arrays are high enough that folks like Google deliberately situate their disk and computer farms in locations like Washington State or Oregon with cheap, hydroelectric power available. Disks have about a 3 year lifespan, so we are talking about $3 Million a year in capital purchases.
On top of this, you'll need to hire at least one, and maybe two full time IT specialists to run the system. That will run you upwards of $150k a year for salary and benefits. So we are at $3.3 Million a year on top of the cameras and the time the officers spend. For a city of 100k population and a department of some 200 officers ($100K a year for salary and benefits) total police budget is probably on the order of $30M a year. So $3M a year out of a $30M budget is about 10%.
Maybe my estimates are off and it is only 5%. That is still a notable line item on the budget.