The $30 CM3 uses a Broadcom 1.2GHz, 64-bit quad-core ARM Cortex-A53 CPU processor, 1GB of RAM and 4GB of eMMC flash storage on a standard DDR2 SODIMM module, making it easier to fit into custom projects. A Lite version for $25 trades the onboard eMMC for space to add your own SD card socket or eMMC flash. Both are available in the UK for £27 and £22, respectively, and convert to roughly AU$40 and AU$35 in Australia.
If you've been working on a design using the original CM1, the CM3 is largely backward compatible. However, the new modules are 1mm taller and run much hotter under heavy CPU load.
Those looking to get started on a new project with a CM3 can get a the Compute Module IO Board V3 (CMIO3). This IO breakout board gives you something to plug the CM3 into for delivering power and processor interfaces as well as HDMI and USB connectors.