You almost want to buy a motherboard when it's cheap and keep it for a few years until the CPUs for it get cheap.That's definitely something that seems to happen with every platform.
Earlier on in their lifecycle, around when equipment starts coming off lease/goes off warranty, seems to me that motherboard prices seem to start falling, while CPU/RAM seem to prices stay elevated. A couple years after the hardware goes out of mainstream use and there's a supply on the used/secondary market, CPU prices start falling and motherboards start leveling off. Then, 8-10 years after introduction, you get an inversion where motherboards start getting more scarce (maybe because there's less of them to start with and they go bad more often than CPUs?) so prices start going up, but there's still a lot of CPUs, so they start going for cheap. Eventually, motherboards get pricy and CPUs are what's cheap. Haswell (X10 for Supermicro) is definitely at that point. Seems like, without exception, Haswell motherboards cost more now than they did 2-3 years ago. As a couple folks mentioned, many motherboards seem to be selling for almost what they cost when they were new. Part of that is supply craziness, but its also partially due to their age.
My guess is a lot of the CPUs & RAM come from highly specialized hardware that have a generally useless motherboard outside of the specific chassis they were in and some places are scrapping the chassis instead of dealing with the space to store them and the problems of shipping them. Hence why there are less motherboards at this stage and a glut of the other components.