These “AOOSTAR” R1 Mini PCs might be right up your alley at $319
It’s running an N100 CPU, 16GB DDR4 RAM, includes a 512GB NVMe SSD, Intel i226-V 2.5Gbps NICs, and has two drive bays for 3.5" HDDs.
For $199 you can buy it without the RAM and NVMe SSD, in case you wanted to customize it a little for your needs.
If you’re willing, these are abundant on AliExpress even cheaper.
I’ve owned a couple and had no issues with TrueNAS SCALE while I was using them.