To answer your first question, perhaps someone who has installed cameras recently can give us a ballpark figure.
I Googled “security camera installation” and it came up with pages of possible suppliers. On entry in particular Security Quotes, gets you to fill in a questionnaire about your basic requirements then, apparently, you get quotes from several companies.
I would think it would be a good idea (and possibly essential) to get Owners Corporation approval on this at a general meeting.
The Strata manager might monitor the camera but would probably charge you extra for doing so. The simplest way, if you don't have a building manager, is to store the images on a computer that's accessible by the secretary or chair of the building.
You might want to ask the strata manager to store the collected images on an external hard drive for backup.
You could even pipe the images into your TV aerial cabling so that everyone can watch (and record). Endless hours of entertainment for all.