It is not that clear-cut. Obviously the optimal case is SMD-only, but there is a threshold when THT-only becomes cheaper than SMD/THT mix. Another thing is that

It is not that clear-cut. Obviously the optimal case is SMD-only, but there is a threshold when THT-only becomes cheaper than SMD/THT mix. Another thing is that power supplies tend to use higher temperature board material (presumably to allow for thinner copper plating) which is somewhat rare and expensive in dual layer configurations.