Chinese labor is a big part, as well as the economics of scale. $78 is wholesale on a Seventy 80 when you buy one, but when you buy 1000 the price drops. The cheaper Celestions are made in China, I'm sure all the other typically expensive components (i.e. transformers) are all Chinese made as cheaply as possible, so they're getting them in bulk and they don't have to pay import duties on them because they're probably all in Shenzhen a stone's throw from each other. I wouldn't bet they're making a lot of money on them when they're on sale, but I'm sure they're still profiting.
I'm literally in the process of doing a layout for a 15W amp right now. I'm going as cheap as I feel like I can get away with for transformers, but that's still $120 just for iron. A set of tubes will run me about $100 so I'm already over the price of this thing, and I haven't even gotten to the PCB, components, hardware, or chassis, not to mention the cabinet, speaker, and labor of putting it all together.
I'm still going to build the amp I'm designing (probably a couple of them) because it's going to be a really cool amp, but yeah I just ordered one of these. Came to $206.58 after tax.