I would argue that LEGO is more expensive, way more expensive.
So what would be a LEGO for adults without the price tag?
PVC pipes and fittings.
PVC fittings are weirdly expensive, considering how cheap the pipes themselves are.
It's probably because the manufacturing process is more complicated.
I bet the pipe is practically continuously extruded from a machine but the fittings also have to be processed and bent into shape before they harden.
I thought it was an Australia thing but yes, $1.50 pipes and $3 fittings are the norm here too. Fortunately, you can 3d print fittings which opens up all sorts of possibilities that don't really make sense for use in plumbing.