I was in this exact same situation, very recently. My overall 2 channel system is right in the ballpark in that overall cost, a bit more but close, (most purchased used). My last emphasis has been upgrading cables, starting with the IC’s, and speaker cables next.I had been running some 10awg basic ‘pure copper’ (yea, right) cables in bi-wire configuration to my Vandersteen’s. They sounded fine, but knew I could do better, and it was time.
My rig is off to the side, and if left that way, I would have needed two 15-16’ bi-wire cables. That length can get expensive given some of the upgrade choices. Shorter speaker cable lengths are preferred anyway, provide many more options, (especially on the used market), and would also reduce cost. Thus, first I made decision to move my old Belles amp to the floor between the Vandy’s. But, that decision made a longer IC from my pre to Belles amp necessary ($$).
First, I didn’t want to spend the cost of my overall rig on speaker cables. After much thought, study, reading options on various manufacturer options, I finally set my sight on NOS Western Electric tinned copper cable. From what I had read, they just sounded ‘right’ *for me*. Found a gentleman who has 1950’s NOS Western Electric 10awg cable, and could custom make an 8’ bi-wire set for me. Luckily, he also gave me a couple options for a 10’ length of IC I now needed, also made with NOS Western Electric wire, (not the same as the speaker wire).
My overall cost was about $440 less tax and shipping. But, that included a 10’ IC cable. If we call the IC $150ish, let’s say my speaker cables cost $300. Is there a difference? Absolutely, in a positive way. Am I pleased? Yes I am. Not only about the sound improvement, but also because I ‘think’ I have cables more ‘in-line’ with the rest of my system, and I ‘think’ it was an overall good investment.
Could I have spent less? Yes, much less on other speaker cable options, but may not have gotten a significant difference over my ‘basic’ 10awg cable. As example, I looked at Canare via Blue Jean for $150. But, I will never know for sure what the actual difference might have been between those and the NOS Western Electric cables I purchased, or even over my old basic copper cables. Or, more expensive Kimber on the used market for that matter. Sometimes these are decisions we have to make a bit ‘blind’, factoring in many different things. #1 being a desire for change. What is right for one person may not be for another. I could have left what I had alone as well. I could have, but most of here on this forum won’t.
But, to your question, it’s pretty hard to identify a certain $X amount of dollars that will improve your system over what you have currently. Only you can decide that.