What's going on with 8700K pricing right now? I know they currently have their supply chain limited by 10nm delays, but $400 is a bit much, especially considering that the 9000-series CPUS are supposed to be coming very soon, with the i7-9700K and i9-9900K likely launching within a matter of weeks. If you are looking for a CPU in this performance range, and don't need it immediately, it might be worth waiting a bit for more information on those. Supposedly Intel intends on announcing them on October 9th, though it's hard to say when they will be readily available. The 9700K will apparently have 8-cores/8-threads (no Hyperthreading) while the 9900K should have 8-cores/16-threads, and cost around $100 more. Both should have higher stock boost clocks than an 8700K, though it will have to be seen how trading SMT for two more physical cores in the 9700K will affect performance in heavily threaded tasks.
Alternately, if you have no plans to overclock, you might also consider the locked i7-8700 (non-K), currently just $310 at newegg. It offers the same number of cores and threads as the 8700K, and boosts to the same clocks with all cores active, or just 100MHz less than the 8700K for fewer active cores, making stock performance very close between the two. It also comes with a bundled cooler, though you will probably want to get a tower cooler anyway, as it won't be able to boost to its full potential on the included one. Of course, the CPU can't be overclocked, but it is currently priced $90 less.