point of diminishing returns

here's a tough one, i know it may have been asked before, but what's the point of diminishing returns for various equpment? where does it start to be all bollocks?
cd player?
amplifier?
speakers?
interconnects?
speaker cables?

ie. what can a R100 000.00 cd player do that a R10 000.00 cannot do? or what can R100 000.00 cables do that R1000.00 cables won't do?!