The robots might be cheaper today. Especially if it is just a webcam, microphone, motorized wheels, battery and the cheapest wireless networking computer being operated by a server in a more secure area of the store.
I imagine a single guard monitoring feeds from ten of these roaming around a wal-mart. The guard doesn't even have to be in the wal-mart. Throw a blanket over it and the guard knows something is wrong and calls physical security.
Throw in some advanced mapping that can compare expected camera images to actual images and use the human to examine positive signals and dismiss the false positives. You might be able to up that human monitor to monitor 100 to 1000 units depending on the false positive rate and the rate of actual incidents.
99% of the time, nothing is happening during security guard duty.