Not $50 and you probably don't already own one, but it can be done on the Uniquiti EdgeRouter Lite ($100, runs Debian MIPS + Vyatta as the default officially-supported Ubiquiti image but very hackable): http://rtfm.net/FreeBSD/ERL/
to get the performance people are constantly raving about, you need the very proprietary binary blobs that take full advantage of the Cavium hardware acceleration.
otherwise it's just another beefy MIPS machine.
Yes, this is an important bit of subtext that most people don't realize.