Why are Harbeth speakers expensive while its drivers are not.

Nostalgia costs, big time. 

Most speakers use lowest cost good enough drivers to minimize price point.

In general the two way speaker that started in the 70's is really one big cost compromise - one less driver but always of limited performance.

The tweeter is made by Seas to spec but I think the Radial2 drivers are custom designed and made by Harbeth.  For what it's worth, Harbeth speakers are like really nice furniture that happens to sounds nice too.  Honestly, I don't think they are worth it new but used they can beat out most monitor speakers in that price range and worth the price. If you're going to buy new and you want that BBC sound, you may want to consider the Spendors. 

Some of the best sounding speakers you could ever own. Lots of R&D and  time into the designs.  Price is based on performance and simple economics. They are worth it is because they sound better then competition at the same price point.  

+1 grannyring!