Believe it or not, it's not nearly as impressive as it sounds.
Most of my cameras are WiFi and the wiring is not run in the walls. My front door camera, for example, is actually an indoor WiFi camera looking out a window with its power cord running through a nearby closet to an outlet near that closet. My living room camera is mounted on a piece of wood that's clamped to a bookcase, its power cord runs behind the bookcase to an outlet nearby. The nice thing about having WiFi cameras is that you can move them around really easily. If I'm worried that one of the cats isn't eating, I can just put a camera looking at the food dish so that I can see what's going on.
My driveway camera is unique in my setup though. It's my only wired camera and it's powered via PoE, so all I need is an ethernet cable. I used an existing hole in the house, where the cable and telephone comes in, and ran the ethernet through that to the outside. From there it goes into some conduit for protection, then gets stuffed behind a piece of siding and run to where the camera is mounted. The mounting is similar to this[0] YouTube video.
It's really hard to say what it would cost to replicate it. My cameras have been acquired piecemeal over the span of 10-ish years. My Blue Iris server is a refurbished Windows 10 Pro (Pro is required so that I can manage it via RDP) business-class desktop machine that cost about $300CAD, Blue Iris itself I think cost around $70CAD. The networking gear is UniFi, but really the only requirements are that the switches and APs are VLAN-capable and that there is some routing/firewall sitting between the VLANs.
Beyond hardware, Blue Iris, and AWS, the software involved all open source. The biggest cost is really time, and it's really hard to put a number on it.
Setting up Blue Iris, tuning the motion detection, and building the Node-RED flows that coordinate it all took quite a bit of time to get working to my satisfaction. Tweaking the motion detection to avoid triggering on shadows from trees in particular is something I spent a lot of time on. It wouldn't surprise me if I spent a total of 20+ hours just trying to cut down on the useless alerts before I gave up and started using AWS Rekognition to filter the alerts. Cost-wise, I estimate that I'll pay about $5/mo for Rekognition once I finish up my free 12-months.
The VPN duties are handled by StrongSwan, I built configuration profiles for MacOS and iOS (using Apple Configurator 2 plus hand-tweaking the resulting .mobileconfig file) to do the connect-on-demand magic. The whole thing is backed by a PKI (internal CA, etc), complete with machine and user certificates for authentication. This whole setup is probably 10-20 hours worth of time.
Typing this all out, it sounds fairly insane, but the knowledge I gained during this process is invaluable. It also took place over a fairly large timeframe, so it doesn't feel like I've invested a lot of time.
[0] https://www.youtube.com/watch?v=UjWkB0UZM1M