i just got my 2006 ZX4 and i was wondering what kind of inexpensive interior mods i could do to it, thanks

You got plenty to look around here to come up with ideas. Additionally, look up eBay, you'd find something.

paint the interior pieces to match the exterior...

Doesnt get cheaper then taht

^^^^^ x2
Change some of the switch leds, and clock etc. to go blue or red or any colour. Always looks sweet.
Indiglo gauges? I fitted some reverse glow gauges just last week and love mine!
Gear Knob?
Lots of little things you can do that add upto a big change :P

We now have core kits available, not shure if it falls under your definition of inexpensive but very nice adition for you interior.

http://www.focusfanatics.com/forum/showthread.php?t=147819

http://www.focusfanatics.com/forum/showthread.php?t=145197