- used HP Elitedesk 800 G3 SFF (4 core i5-6500, 8GB ram, 240gb SSD, 4x PCIe slots) $170 shipped
- 2x new dual 2.5Gbe PCIe cards $40 each
- 1x used quad port Internet gigabit $30 shipped
So for $280 I have a machine that will route at 2.5Gbe for a few machines and gigabit for the rest of my network while using about 25 watts. If you don't need that many ports you can cut the cost down considerably with a smaller machine like a Prodesk 400 or 600.
I'm using VyOS but OpenWRT, Untangle, OPNsense or Sophos Home would also be perfectly fine choices.