Some larger capacity drives, like 1TB, are cheaper than lower storage drives, like 64GB. This can be due to low sales volume for such reduced storage at this stage of computer development.
Or it could be that reduced capacity drives may also be made physically smaller, like a SATA DOM. DOMs, both SATA or USB, can be made to be installed inside a server on the actual server board’s interface connector. Thus, a 2.5" SATA SSD would not fit that requirement. And yet a SATA DOM is low enough sales volume to cause it to cost more.
Here is the reasoning behind TrueNAS’ decision to have dedicated boot drives;
I have to waste an entire drive just for booting?