For those who wonder why some lenses cost more than others, here's a look behind how Leica does it. For those of you who are Canon fans, you might want to set some time aside and take a look at how Canon assembles the 500mm https://cheesycam.com/wonder-why-lenses-cost-so-much/. Imagine all the other 'L' lenses...
These videos are long, but super super interesting for anyone who cares anything about lenses. Most of the time I feel like everything camera related is overpriced, but after you understand the work behind creating some great lenses, you'll feel very differently (well at least for Canon Lenses). This is a three part series that covers the making of a Canon L series 500mm F/4. Take the time to watch when you can.
Part #1 Making of Canon 500mm Lens
Part #2 Making of Canon 500mm Lens
Part #3 Making of Canon 500mm Lens