That's the same type of setup that is common for security cameras using PoE. It seems to work well. Depending on how resource intensive the video analysis is, you could have several cameras being analyzed by a single mini computer (Atom based $100). That might be slightly cheaper than a single Pi per camera.
Also, where are you finding those PoE switches?! I was looking for 8 port ones just a few months ago and they were about $50 for cheap ones and at least $80 for brand names.
I just did a quick look upon Amazon. Was looking at 5 port ones so maybe sweet spot price wise. Had quick look at 8 port ones and branded would be around prices you have - though Netgear seemed closer to 50 price wise. Do note many 8 port models only do POE over half the ports I've noticed, so mindful of that and two 4/5 port affairs maybe better price in some situations.
Though if you're in no rush, January is sure to have some good sales/warehouse return deals. Though if you want to go offbrand and 10/100 do you then "Cudy FS1010P 8+2-Port 100Mbps Ethernet PoE+ Unmanaged Plug and Play Switch, 120 W, 8 10/100Mbps PoE+ Ports" on Amazon will pull up a 40$ model as an example that seems like maybe worth checking into.
Also, where are you finding those PoE switches?! I was looking for 8 port ones just a few months ago and they were about $50 for cheap ones and at least $80 for brand names.