Can always roll your own card.
ATMega640, $11.80 (1-lot,digikey) (smaller memory version of $18 ATmega2560)
Support pieces - depends on your IO needs - crystal, caps, 10K resistor, 100nF caps - <$1
MCP3201 - $1.73 (1-lot, digikey)
Board - $2.50/board (10-lot 10cm x 10cm, iteadstudio)
ADC signal conditioning?
Serial buffers?
Headers?
Power regulation, wallwart?
Provide more project definition.
In the meantime, can prototype with ATMega640 using a board like this
http://www.schmartboard.com/index.asp?page=products_qfp&id=70
(0.5mm pin pitch)
May need additions to boards.txt, avrdude.conf, pins_arduino.h,
or create new "variant"
4 UART - one is USB, one RS485, two are TTL serial?
USB/Serial adapter is another chip
RS485 buffer is another chip
Ethernet, Wifi, more chips
How do all that and keep cost comparable to an Uno with its $2 '328 processor?