There is no hard rule. Especially when it comes to budget.
If a 500$ power cord makes your 1000$ DAC sing like never before, use it. Or, if you favorite cable on your 5000$ DAC is a 30$ cable, well, so be it. only the result matters.
I use super cheap amps (tiny class D and tripath amps, in a multi-amp config) hence my cable budget is far above my amplification budget. So what? It works (very well) for me. Listen to your ears, not to the gurus.