CPUs aren't as important as GPUs for gaming. If you just focus in on upgrading your GPU, and you get the top of the line shyt, you're looking at like $1500-$2000 every 4 years. Then when there is a major CPU upgrade, which might be every 5-6 years, you'll need to spend kinda a lot, because you'll need to get a new mobo because the socket always changes. Then you might need to get new ram as well. So maybe another $2k for all of that. You can keep the same case, ssds, psu (if it is good) and stuff if you want.
Real top of the line builds can cost up to $10K. My last build was $10K, but I had crazy custom water cooling and the like. My current build, which also uses all top of the line stuff (custom water cooling, custom cables, expensive water blocks, etc.) was around $8k.
Then, you should also get the best monitor you can afford. That could cost up to $2k, but realistically it'll probably only be around $1K (maybe even less).