It's absolutely that high. Upgrading a Mac Mini from 256GB to 2TB is an extra $800, and a high-end 2TB NVMe drive like the WD SN850x is around $150 at retail. Even the 8TB version of that drive is only $650.
That's why external SSDs are so common in Mac setups, even accounting for the additional cost of a Thunderbolt enclosure it's usually still significantly cheaper than getting a bigger internal SSD from Apple.
That's why external SSDs are so common in Mac setups, even accounting for the additional cost of a Thunderbolt enclosure it's usually still significantly cheaper than getting a bigger internal SSD from Apple.