Can anyone suggest some reasonable-but-cheap hardware solution for an openbsd firewall "appliance"? I've been using old PCs for years but they are not really cheap when you consider the power consumption.
Soekris hardware is often suggested, but they are not so cheap.
If you want something router-like, PC Engines apu1d and apu1d4 at $124 and $145 USD are good options, as is the older (but slower, i386-only) alix2d3 at $103.
There are also plenty of micro ATX or all-in-one fanless PCs with dual-NICs in a similar price range. Or dual-NIC Intel NUCs, as well (not fanless).
Soekris hardware is often suggested, but they are not so cheap.