I'm currently building a simple clock-in system where cards are logged and authenticated with a central server.
I have prototyped this with a mega, RFID reader and a 6100 based display, however I'm now looking at producing a small volume (10-20) to actually be put in to use.
The best solution I've found is an Arduino Ethernet, but if I could get ethernet on the Mini Pro reasonably easily and cheaply, I think this could be a better solution.
Cheapest may be a simple W5100 brick like:
And a Pro-Mini copy like:
If you asked for a quote on 20 of each (You wouldn't need the programmer duplicated for the ProMini).. It would probably be $35 or so..
But a better alternative would be an "Iboard" with built-in Ethernet which would probably be $28 or so total at those quantities.. I will check price..
DISCLAIMER: Mentioned stuff from my own shop...
Something like a nanode might be an option:
I think they use a different ethernet chip so at least you would need a different library from the W5100 based option.