JLCPCB have changed the game. Five 2-layer PCBs of up to 100x100mm cost just $3.50 including global shipping. Things get more expensive if you stray from their standard specs, but you're still looking at just a few dollars per board.

https://jlcpcb.com/

The biggest electrolytic caps in this circuit cost $3.29 each in qty 1, but they're fancy "audio-grade" Nichicon caps; a standard-grade capacitor of that size would cost you $1.68 if you want a Japanese brand, or as little as $0.36 if you can settle for a Chinese brand.

I think the biggest cost is labor. Dumpster diving for speakers and then spending dozens of hours in high skilled labor to replace the insides is actually hilarious. I wonder how much he'd charge a client for a project this size? Probably many thousands of dollars.

But I can't knock a man for having a hobby. Clearly they're optimizing for fun and nerd cred, not cost.

A PCB like that would cost around $1 each, if you got 10 or so, so it's not expensive at all. I don't know how much assembly costs, but I'd be surprised if the total was over $20.