The "tank" you are referring to is most likely an HRV or armored personnel carrier. The department near me has one as well (actually two now). It is not a tank as it doesn't have a cannon on it. It is used to safely move officers up to a location without them being exposed. It is used in almost every barricade situation. It also has a ram on the front that is used to assist in breaching the front door (or even windows) on a house when they need to make or are getting ready to make entry (or send in a robot or gas/flash bangs). As for cost, in most cases, the departments don't pay for them as they are part of grants or "donations" from the Federal Government.
With regard to helicopters, the same department has two as well. They cycle them in and out so one does not get too many flight hours in order to keep it in good shape. For this department, the helicopters are used for medical as well. In fact, while the people on the helos are police officers, they are fully trained paramedics and their primary mission are medical calls.They will even break off of a police call to respond to a medical call.
So, yeah, the body cams are many times something that breaks the budget compared to those toys you mention as they are just a small cost with regards to maintenance, but with body cams there is the entire initial cost of setting up the program and then the continued maintenance cost, which will go up exponentially every year.
- Merg