Ubiquiti ERLite runs Vyatta on Debian on MIPS. $100 and it has TCP + VPN offload and 3 real interfaces. It might be possible to run PFSense on it; I've installed PFSense to a CF card on an IDE adapter running in a thin client (you can also find these on eBay, some with PCI-E even...check out the HP T5745 for example). You can find a lot of thin clients on eBay for under $100 if that's what you want, and most are fanless and low-power. The only issue is fitting multiple NICs, but again, PCI-E if you know where to look.