The lower-end PFSense rebranded hardware [1] seems pretty competitive to me, with the $300 box they're expecting to release at the end of June looking especially nice in comparison to the soekris boards. Alternatively, if you're willing to spend a few dollars on RAM, there are a number of barebones systems out there with Atom C2[3,5,7]58 processors that will give ten times the performance, so you could do some file serving with the box too.

[1] http://store.pfsense.org/hardware/