We followed this tutorial to make them and spent about $13 - $14 on each one, which was cheaper than buying an Arduino Uno at the time.
Now you can get an official Arduino Uno for $24.95 , so let's see if making your own is still cheaper.
Here is my current part's list.
15405 - .22pF Capacitor 50 Volt 08 x 10 = .80
333201 - .12 Green 3mm LED x 10 = $1.20
51262 - .5 Volt Regulator 25 x 10 = $2.50
94212 – 10uF 25V Capacitor .19 x 10 = $1.90
112299 - . IC Socket 25 x 10 = $2.50
119011 - Reset Button .29 x 10 = $2.90
137891 – 16 MHz Crystal .35 x 10 = $3.50
2129334 – Atmega 328 with bootloader $4.39 x 10 = $43.90
159611 Power Socket 1.75 x 10 = $17.50
Perfboard = $1.25 x 10 = $12.50
$72.20 / 10 = $7.22 each
So you can now make a stand alone Arduino Uno for about $7.10. I haven't included hookup wire or shipping costs and in my previous version I did use really nice Adafruit protoboards which are more expensive than perfboard.
You can make it even cheaper buy buying the Atmega328 without the bootloader or by not using a power socket. One last note is that in this scenario I was using an actual Arduino Uno to program the chip and then remove it and put it in the stand alone Arduino.
Making your own is nice as you can throw it into a project and add or take away features you don't need. It is also easier to solder other components or wires in place instead of having wires come loose from female header pin sockets.