I was curious, and last night I priced out a basic "stick pfSense on me" box with reasonable quality components. With the exception of Realtek NICs instead of Intel—which might be a problem as you go past 150Mbps, Realtek NICs don't have a terribly glorious reputation—you can assemble a Mini-ITX based system with mirrored drives for $360. Intel used to make some dual-NIC "corporate workstation" boards that worked really well, especially if you ponied up for a better CPU that supported vPro, so you could do remote IPMI console. Unfortunately, Intel got out of the motherboard business.
I haven't tried any of this equipment, so it may actually suck, but here's the bill of materials I came up with for "so you want to build your own router with commodity parts". Obviously, you could go with server-grade parts or with a ready-built box of various flavors too...
- BIOSTAR Hi-Fi B85N Mini-ITX motherboard
- 2x4GB DDR3 1600 (PC3 12800) DIMMs
- Cooler Master Elite 110 RC-110-KKN2 case
- COOLMAX CX-400B ATX power supply (but I'd spend a little extra on an Antec VP450 myself)
- Intel Celeron G1840 CPU (dual core 2.8GHz Haswell)
- Two Western Digital Blue WD2500AAKX 250GB disks
Something like that should be able to handle any reasonable real-world home network needs. RAM is pretty cheap; you could probably do fine with 4GB. SSDs are all the rage, but spinning rust is cheaper and disk speed isn't really a big factor for a router.
However, as a matter of common-sense security, I'd recommend keeping any such box limited to being a router/firewall. Sure, run DHCP and DNS services on it... perhaps OpenVPN... but resist the temptation to load it up with other services. You'll just bog down the performance and increase the potential attack surface, especially if you accidentally misconfigure the firewall.