Bajdi blogged about his Arduino pro mini undershield:
I’ve designed another PCB :) This time it’s a simple undershield for the Arduino pro mini. I received the PCB’s from Electrodragon (12$ for 10PCB’s) last week. The PCB has a schottky diode and a 5V linear regulator, I’m using an LM2940 5V regulator. For the rest there are just 2 rows of female headers to plug the pro mini in and 3 rows of male pins on each side.
The reason I’ve had this PCB made is that pro mini’s are dirt cheap these days, you can find them on Ebay or Chinese shops for mess then 4$. That makes them the cheapest Arduino on the internet. It’s cheaper and easier to buy a pro mini and integrate it in to your own project then to design your own PCB with an ATmega328.