You can look at the manuals on the Sig website.

The 1/6 cub kit is mostly a bunch of sticks, with
a few die cut parts like ribs. Not too expensive to
produce.

The 1/4 cub is also a bunch of sticks, but the
sticks are bigger and longer, so the kit costs more.

The 1/5 cub is mostly laser cut sheets. More
expensive to produce (probably not that much
more expensive.) It is much easier to build
which makes it more desirable. The folks
at Sig think those who desire it are willing
to pay a lot for it. They may find that
many people do not desire it that much or
maybe that they do.

Jenny