Tube Amp Pricing

The schematic design of an amp drastically changes the sound. I prefer the sound of single ended amps, although have also enjoyed triode & pentode designs. Also, even though (in some cases) the basic components appear to be the same, there are different levels of quality with them. For instance capacitors vary greatly in cost depending on brand, as do transformers, resistors, tubes, & wire. All of these can change the sound of the amp. 

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.