I learned recently that brass has about 1/4 the conductivity of copper. This thread suggests any “copper vs brass” mental anguish should be “left by the side of the road” as the diff is way too small to “knot up the knickers.” SQ?
If you have learned this, then it is only a small step to calculate the differences when using different material (or "quality") of BP and different cable diameter and length - there are online calculators for this, so you don't even have to learn the theory.
Let's consider, for fun, the effect of using iron instead of copper on a BP. We assume that the contact area of the BPs is identical and so large that this has no decisive influence.
Let's assume that the BP has a length of 5cm and a cross-sectional area of 2.5mm² (this corresponds to standard speaker cable):
R=ρ*l/A
R [Ω] resistance
ρ [Ωmm²/m] resistivity
l [m] cable length
A [mm²] Wire cross-section
p copper 0.018 Ωmm²/m
p iron about 0.12 Ωmm²/m
Iron has about seven times higher resistivity than copper - so an extremely poor material for our BP.
So for our copper BP we get a resistance of 0.36mΩ and that of the iron BP is about 2.5mΩ.
For comparison, an extremely thick 6mm² and with 3m very short copper speaker cable has a resistance of 9mΩ.
A more realistic comparison would be a 4mm² speaker cable with 6m length, which would have a resistance of 27mΩ.
Many will use only 2.5mm² speaker cable, or even smaller cross-sectional area, then we would already be >40mΩ.
Now everyone must decide for themselves how much money to spend on BP and speaker cables, and how much to value the possible effect.
In the realistic example, 4mm² speaker cable with 6m length, even if iron nails are used as BP, the resistance deteriorates only by 10% (compared to copper BP).