I also own a Mazda 3 and was thinking of getting this iPod unit. It's $119.95, which is pretty reasonable.
http://www.mazda3forums.com/index.php?topic=34982.0
Plugs into the back of your headunit and charges the iPod while it plays. Also keeps your display working and your stereo controls. The cord will come out of your ash tray, so it's a really clean install.