I was looking for a cheap one port serial console server I could use to monitor a server far away. Turns out they are quite expensive, so I looked at making my own.
I ended up being told to look at a WR703N which can be had for around $25, either from china, or sometimes a US location like amazon . Another option is the mostly identical TL-MR3020
Both have an openwrt distribution which you can get from http://wiki.openwrt.org/toh/tp-link/tl-wr703n and http://wiki.openwrt.org/toh/tp-link/tl-mr3020
One way to get serial could be to open the device and with a bit of soldering and a level converter, one is supposed to get a serial port off the bare chip. That was more time than I was willing to put in, so I just used the built in USB port and a PL2303 USB-serial converter.
Unfortunately, the USB support on those devices has a bug there full speed USB devices (i.e. USB1) do not work reliably and in the case of a serial converter, it'll hang after a minute of use or two.
This is described in a long thread at https://forum.openwrt.org/viewtopic.php?id=39956&p=4 but you can jump to post #84. Basically the fix is to use a passive high speed (USB2) hub, and plug your USB1 device into it. No USB power required, and a cheap $10 USB hub works fine.
Next, get an old USB key, format it with ext4 as explained below, and edit /etc/rc.local on the router to mount the USB stick. This allows logging the serial session without writing to the main flash of your device, since that's not replaceable.
Ok, so the end cost is between $40 and $50, but you get a super small linux capable router that acts as serial console server, can log the output and provide ssh access. Still not bad... (I used a free 64MB USB key I had lying around).
# Get the card ready with
# mkfs.ext4 /dev/sdc1
# tune2fs -c0 -i0 -m0 /dev/sdc1
# opkg install kmod-fs-ext4
mount -o noatime,commit=600,data=writeback /dev/sda1 /mnt
# opkg install kmod-usb-serial-pl2303
# opkg install screen
# This will dump serial logs in /mnt on your USB key
screen -m -d -L /dev/ttyUSB0 115200
# If you don't want screen, you can try minicom or microcom