I've found over the years that it goes in cycles, sometimes the self
build is a cheaper option and sometimes the off-the-shelf ones have
the edge. A lot depends on the specification of system being built and
the higher you go the more you can save with a home build but the
un-accounted costs are the time researching component and the actual
build itself.

It's worth checking out special offers from component suppliers too as
often they have slightly lower specification systems, whether full
builds ( minus OS ) or bare bones or motherboard bundles, that they
need to shift and offer at a good discount to do this quickly. Done
this a few times when building for customers and the savings can be
significant. Again it depends on the intended use, you aren't going to
get cutting edge in the bargain basement bin