Ha, well I am not sure on the exact price.
Surely they are getting a bulk discount.
I'd estimate cheaper in cost than anything we are used to identifying as a "hifi" product.
(Between $0.05 and $0.10 per foot)
The cable that came into my house was very cheap looking, thin, cheap feeling and truly not impressive. Yet it carried all the goods flawlessly, all that data and the net was smoking fast.
Think about it. Incredible technology.
Anyway
Actually you should not compare these technologies to much. They both use light, and both send the light though a thin translucent cable. It about stops there. The stuff that comes into your house is actually glass and a much higher quality fiber compared to the plastics that are used for Toslink. Now surely some of those expensive Toslink cables might also use flas fibers, but that's mainly to try to justify the price.
Besides the materials there are many more differences in modulation and specific properties of the fiber itself, all of them were devised to increase the data capacity and range of the cable.. where you can go upto 400 Gbit/sec vs ~4.5Mbit/sec for Toslink on a single fiber.