The Raspberry Pi Zero was supposed to be a $5 single board computer. However they were quickly bought up and resold for $25 or more. With the introduction of the Raspberry Pi Zero camera edition the prices are now coming down to around $18. So I bought one and a kit to be able to use it for another $15. To be able to use the Zero you will need a micro SD card with NOOBS on it, a 5V 2 Amp AC adapter, a mini USB to regular USB adapter/hub and a Mini HDMI to regular HDMI adapter/cable. A case to hold it in helps a lot too. To be able to use the GPIO pins you will need to solder in your own 26 or 40 pin header.
This first picture is of the Raspberry Pi Zero camera edition.
This picture shows the Raspberry Pi 1A, 2B, and Zero next to each other.
This third picture shows the Raspberry Pi's in their cases. I have a model 2/3 case on order from china that has not arrived yet.
As far as cases go the case on the left broke off one of the corners. The hole for the ribbon cable does not fit the ribbon cable connector. I was not happy with that style of case. The case for the zero consists of two layers of Plexiglas and that style of case is what I ordered for the Raspberry Pi 2.
Here they are all in their cases.