I view everything between the source and the speakers as 'cables'. Still, 30% the cost of the entire system seems close to the upper limit to me. But that's approximate, and when you buy used the proportion might be different. Besides, cables are easy to buy and sell, sometimes hard to find , though.
What's your investment threshold?
This question only goes out to those like myself who enjoy the system gains running better cables bring. Non-believers can save their money and comments for another thread.
Do you impose on yourself a percentage range for cable spending relative to other components?
Speaking for myself, I do consider cables to be a component decision worthy of budget, but I seem to have stayed in the 10-30% range of what I've spent on other components. I can't see spending $1k for a cable on a $1k component for example. I have spent $1,500 for speaker cables for a $6k amp. That seems nearer the high end of my range but it still fits.
I realized I haven't planned this as a hard rule, but that's kind of how this has worked out for me over time. Anyone out there think a much higher percentage is justified? I won't argue because these improvements are worth all the enjoyment you can get, so more power to you.