So despite the rising prices of the hardware in the recent years, we still get double the performance for the same price for the last 5 years, or about 20% per year. Though, admittedly, most of that is in fact just from the last 3 years, so about 30% per year then? Still after all the hate the FX series got through the years, we can say it aged not as bad as expected, at least if we are looking at the top part of the generally available series (as there were FX-9370 and FX-9590 with 220 W TDP). On other hand the Radeon R9 390 was a popular card back then and the 8 GB frame buffer seems to come handy these days. It is still competent card for 1080p and onlne gaming, probably comparable with the likes of Radeon RX 570/580 or GeForce RTX 1060.
But at the moment it is not very advisable to go for them if you buy second hand, as you can find Ryzen 1st gen and you can even buy new Ryzen 5 1600 for under $100. Unless you get it dirt cheap for a (nearly) complete system.