Even at 400 hash you get a CPU that pays for itself within 6 months at current prices. Since you need a CPU to mine with your GPUs anyways why not get one that pays for itself, brings in a little profit after that and also has good resale value?

For those that have an 8 core Ryzen and have not been able to reach 600+ hashrate on XMR, you will need to enable large page files (requires Windows Pro). Once I did that I went from ~450-470 to 600-630 hashrate on my R7 1700X @ 4.0 ghz.  It's a nice boost in AEON too (the most profitable CPU coin right now).