As for this magical ratio, well as @mikapen said sometimes spending more = worse sound. Not all companies are like Harbeth or ATC where spending more gives a better sound. Far from it, especially for amps/cables etc. But we all know that, don't we?
Again, it all depends upon at which price point you can hear the performance flat lining.
You might have to discover where the sweet spot in a particular range might be, or else risk being forced out of your listening room by the sheer number of boxes which have almost surreptitiously taken up residence.
I have a friend whose front room looks like he's installed a console from Cape Canaveral. Seriously, reconnecting or rewiring his receiver is a two man half day job.