Actually, that's among the cheaper sets of 16Gb ddr4, there's not all that much difference in price going lower. Ryzen cpus (unlike intel) get considerably more use out of ram speeds, there's upto @ 20% difference between 2133 and 3200, which is huge comparatively. Right now, there's still some compatability issues with Ryzen and ram, but that's mainly in those running SkHynix (Corsair) and Samsung B-die has no issues (G-skill). It's only above 3200 that ram prices get ridiculous, so it's a balance of what's the best bang for the buck. The MSI B350 Tomahawk is a full size ATX, not mATX and is similarly priced (bit more, but not much). The Asus Prime series is also good, but ATX boards cost more than their mATX.
The biggest difference between mATX and ATX is in the 3" of board real estate the ATX includes sli/crossfire options and 2-3x more fan headers, generally, but otherwise the boards are basically the same. MATX for most ppl tend to be more versatile since case choices are wide open, mATX or ATX, full towers, whereas ATX are stuck with ATX or full.