Background
Apparently an OEM rear diffuser has been launched in the USA. You can view this thread on evolutionM: 08+ Lancer OEM Rear Diffuser.
Here are some shots they had if you're lazy.


Those retailed by Mitsubishi in the US based on the thread go for US$172 or A$208 (delivered).
RoadRace Motorsports also has a rear diffuser as shown here:

They go for US$199.00
What I'm offering
I have now manufactured in China a copy of this CF rear diffuser:

and will retail it between A$150 - A$200.