I'll go as high as £550 (so $600ish) for my gaming rig but the performance has got to be there. I'm really looking at the platform en toto, as my next real upgrade is going to be Skylake-X or Zen. That's not to say that I expect Skylake-X performance from Zen, but rather that whichever provides the right features at the best price/performance will get my money. Zen's scalability and the unity of the AM4 platform means that we should be able to see 42+ PCIe lanes, quad-channel RAM, etc.
Modern platform features and Haswell-ish IPC with a mainstream i7-k price would win me over but I see that as a best-case scenario.