What's your investment threshold?

I've never gone to the trouble of limiting, and therefore, quantifying cables as a percentage of system costs. I just buy what I think will work best. Doing a quick and dirty computation, cables come out to about 33% of total system costs. 

Add in the many cables I've boughten, it gets a lot worse, so I just factored in the contents of my present system. I don't regret it one bit.

All the best,
Nonoise

Do you impose on yourself a percentage range for cable spending relative to other components?

No. I don't crunch numbers, I listen to music.
That doesn't mean that there aren't cables that I can't afford, but I don't look at any system percentages when buying, only $$$ values.

I have spent $1,500 for speaker cables for a $6k amp.

Does $1,500 speaker cables count as compared to your amp, or your speakers, or both?

As far as cables go I pretty much always buy used and have learned there is probably less correlation to $$ and great sound then with other gear. Plenty of relatively speaking affordable cable can sound great in a suitable system. Also there are so many great deals on used cables its easy to buy and try and resell and try another. I have personally maxed out at $6-700 but for cables that retail anywhere  from $15-2500. And I have another one on the way, I must be sick or something :p

In my opinion, one should think about an audio system holistically. If you love it, the cost is not stressing you out and detracting from your life, and it looks and sounds the way you want for the music you listen to, it's all good.

This hobby is for my spirit. Laying down a hard rule puts a shadow over the whole endeavor, so I won't do that. I will, however, judge the cost/benefit for a component (and cables to me are components) too high and decide it's not worth it. Once I cross that personal threshold, I start to regret spending the money and feel like I've been taken, neither of which is a desirable outcome.

However, through this process I've come to learn that for me spending 100% of an electronic component's value is where the benefits really taper off. For example, I own an Arcam CDS27 CD player. The dealer who sold it to me insisted I have a "modest" system and therefore I should buy modest cables. He literally made threats in a joking way and also touted his decades of experience. However, I borrowed the cables he recommended and they didn't work for my ears. 

I finally ended up with a Nordost Tyr 2 RCA interconnect on the Arcam CD player. By trading in my old Heimdall, I was able to get into a 0.6 meter cable for $1,575, or about 100% of the cost of the player, and it sounds fantastic: natural, open, relaxed but also dynamic when it needs to be. Exactly what I wanted. If I'd settled on a lesser cable, the whole system never would have gotten "there" for me, so essentially it would have been wasted money. If I don't like how a system sounds or it's fatiguing, I tend to not even turn it on. 

I'm currently running the Tyr 2  interconnect with Kimber 8TC cables that I bought as a stop-gap because I was spending a lot of money at once, and while they are bit muddy, I like the tonal balance. I tried Heimdall 2 speaker cables and found they accented high frequencies too much and gave a kind of punchy, night club type of bass--that combination wore on me through extended listening sessions. So, I'm thinking of buying Kimber Monocle XL speaker cables and if that doesn't get me "there," looking for a used pair of Nordost Tyr 2 cables down the road. 

I know that with Nordost Heimdall 2 and Frey 2 make the most sense for my "modest" system, but it's really about what gets you there. As long as you can afford it.