$100 for the 10GbE upgrade doesn't seem unreasonable to me.
A PCIe 10GbE NIC using the same Aquantia chipset as existing Macs is $80-$90. https://www.amazon.com/ASUS-XG-C100C-Network-Adapter-Single/...
edit: in fact, compared to Apple's usual price gouging for extra RAM/SSD/etc it is downright reasonable.