You can look at an ISA serial card with, say, 4 ports. You just have to see what chip is used and if there is a current working driver in the kernel. Whack on 'mgetty' and set up pppd on a per-port basis, plus maybe dhcpd, and your other computers can connect as if they were just connecting to an ISP via dial-in - except that you're the ISP and you don't need modems.
Admittedly that hardware may be a wee bit difficult to come by, but some contemporary boards have that sort of stuff - for example, check out the "PPM-GX" from WinSystems (
www.winsystems.com). The only problem: that board costs a bit and using it as a serial router is waaay overkill - you might consider it if you want to set that device as a server+router and your older machines could even connect as "thin clients". I can't imagine such a purpose built computer coming in under $400 - and that's assuming you can get a main board for under $200.