If this was a PC then a 1TB SSD would be about $200, but the 2018 Mac Mini has a non-upgradable soldered-on SSD on the motherboard so you have to pay Apple's build-to-order prices if you want one. (The RAM is technically upgradable but requires a security Torx bit to access just to be annoying.)
It's accurate maybe not according to the market in general, but it's how the pricing on the BTO works. I tried building a similarly specced Mac Mini just now and upgrading to the 1TB SSD is +$600, whereas 2TB would be +$1,400.
The 970 EVO performs noticeably better in almost all benchmark categories (latency, sequential read/write, 4K read/write) than Apple SSDs. The 970 EVO uses MLC.
> which would cost $2499 (much of which is the SSD)
A 1TB SSD is maybe $200 retail?