In 1974, Intel released the 8080, which basically was the first microprocessor good enough to build a serious computer around. That led to the Altair 8800 the following year, which was the beginning of the personal computing revolution.
What sometimes gets forgotten is that later in 1974, Motorola released the 6800, which while not quite as good as the 8080 (in speed or code density, the latter being perhaps even more important in those days), was the second microprocessor good enough to build a serious computer around, and accordingly likewise someone did the following year: http://www.computinghistory.org.uk/det/16739/SWTPC-6800/
It's basically the same kind of design as the Altair: horizontal box with a backplane that takes expansion cards that are expected to hold most of the actual hardware. It doesn't list separate prices for kit versus assembled, so presumably the listed $395 (from the above link) is for assembled, compared to $621 (from Wikipedia) for the Altair.
Why the price difference?
It's not because MITS was overcharging for the Altair; on the contrary, they were drastically undercharging, to the point where they couldn't expand to meet demand and there was no margin for retailers.
It's not because the inferior CPU was cheaper; Motorola matched Intel's price for their CPU. (https://www.techspot.com/article/884-history-of-the-personal-computer-part-2/ "Roberts was able to secure supply of the 8080 for $75 a processor (Intel's list price was $360, as was Motorola's 6800)...")
The SWTPC does not have the front panel with the switches and blinking lights of the Altair, which would have saved some cost, though that component doesn't feel like it should have been very expensive.
Perhaps more significantly, it uses the SS-50 bus compared to the S-100 bus. That's 50 fewer wires on the backplane, 50 fewer pins on each edge connector. Intuitively, it feels that should cut the cost significantly, though it is said that MITS was able to source 100-pin edge connectors cheap as military surplus: Why did the Altair use 100-pin edge connectors?
I am curious about several things regarding this machine and the 'might have been' branch of personal computing history that it represents, but to ask a specific question here:
Assuming the price difference reflected actual manufacturing cost difference, what accounts for the SWTPC being cheaper to build than the Altair?