Tube Amp Pricing

R&D, low unit volume to amortize the fixed costs, and piffle (aka, audiophile tax)

audiophile gear is subject to some concept in economics called reverse xxx - as the price goes up, sales go up

the classic example is BMW cars in the 1980s where sales were fueled by yuppie desires for showing off a luxury product (in the early to mid-1970s, my 2002 was relatively unknown and priced about the same as Olds)

somewhere I saw a video of Audio Research amp construction - maybe a factory tour - it shows how they are built (laboriously), the careful selection of wire & components, and the testing of each unit

Harman Intl. has an extensive laboratory used for audio testing (but are a very large conglomerate these days so it is affordable)

those 2 examples show good reasons why some things cost a lot

There's many way over priced tube amps, just as there's many over priced speakers, cables, dacs, transports...you name it. 
Like much of high-end audio, the price doesn't always reflect the quality and performance. Much of this industry is built around the sucker, and as they say, there's one born every minute.

There is probably an asymptotic relationship between price and performance. Some circuits are simple (ie., SET) and use the fewest components; others are much more complex. Individual components then become a factor - silver wound transformers, for example. Then there is the economy of scale.

Wow thats a tough one its the quality of the parts and construction of the unit also were it is made that helps determine the price.

While there are plenty of tube amps that one could call over priced the reality is it's hard to make a truly good tube amp cheaply. Parts cost can be quite high transformers in particular can be quite costly and are critical to an amps overall quality. Other parts such as capacitors and wiring can also be rather pricey. Basically  in an amp with fewer parts the quality of those parts is even more important. Also the relatively beskope nature of these amps makes them that much more expensive, parts are bought in small quantities and cost more. Sometimes even vintage or very specialized parts are used. Factor in build time labor and a solid markup and you start from a fairly expensive "floor" cost. It's when you start seeing glitz and glam on amps and marketing hype and technobabble that prices start to skyrocket.