In 'Lies, Damned Lies and Benchmarks' (2014) I wrote about varying measures of CPU power versus perception of speed, and concluded that there were no order-of-magnitude differences in affordable CPUs. Three and a half years later, there ought to be some change in the market. What's the best bang-for-the-buck these days?
I characterized an AMD Athlon 5150 as the low end: x86-64, 4 cores, 2GHz, and for a cost of $50 or so, a single-thread performance index of 804. For the top of the reasonable range, I selected the i3-4470, 2 cores producing a benchmark of 2240 each for $150.
The low end has moved a little. A $50 Intel Pentium G4400 scores 1878, more than twice as fast at the Athlon. You can get slower, cheaper CPUs soldered into motherboards; the market doesn't really have a place for swappable PC-class CPUs under $40. On the other side, you can pay $140 for an AMD Ryzen 5 1400 (4 cores, each at 1726, plus 4 more hyperthreads) or a few dollars more for an Intel i3-8300 (4 cores at 2300, no hyperthreading). Essentially, you can get two to three times the performance for the same price, but only in parallel -- the single thread CPU speed hasn't changed appreciably.
The cost-no-object fastest single thread CPU is the Intel i7-8700, which will cost just $350, heat up your system with a reasonable 95W, and score a charming 2700 on that benchmark. You do get six cores and hyperthreading, but if you're going for a parallel workload, you might be better off paying $25 less for a Ryzen 2700X, 8 cores, 16 threads, and a score per thread of 2200. Depends on your workload, but it doesn't depend that much.