Why are Harbeth speakers expensive while its drivers are not.

The simple answer is if they sold it at a lower (or higher) price, the decrease in revenue would not make it economically feasible to produce the speaker.

Selling at a lower price decreases revenue and creates a higher demand. Selling at a higher price decreases demand and can result in lower sales, especially if there are competing products at that price point. As an example, if you sell 100 widgets for a dollar, the revenue stream is $100.00 which covers labor, overhead and profit. If you decide to increase profit by raising the price 10 cents, that may result in selling, say, 90 widgets and the total revenue is $99 which means a loss of a dollar. Or your sales can collapse completely if there are multiple widget competitors who kept their price lower.

Setting the price to cover expenses and make it worthwhile to produce a product is tricky, especially when there is fierce competition for the consumer's dollar. 

I'm a Harbeth lemming.  They just sound right to me.

It's a business.  They sound great. They have overhead taxes etc. When you develop something you should charge for it! 

I currently use the P3SER, and they are superb. I bought them used, as I almost invariably do with audio components, and am quite comfortable with the value that they represent in that context.

Roxy54, perhaps you are thinking of Doug Schneider of Soundstage.com. I am Doug Schroeder of Dagogo.com. He did the review of the Harbeth 30. I have done no Harbeth reviews.  Our identities are confused by others fairly often.