Here is a quick breakdown of the costs involved in building the Arduino. I got these parts from my local Radio Shack because I didn't want to wait for shipping so you might be able to do a little better if you're willing to wait for delivery.
Arduino UNO board: $30
Jumper Wire: $7
IR LED: $3
I would recommend investing in a case for both security and appearance although it is not required for functionality. Here's one from Amazon for $12:
So all told, it's about $50 in parts to essentially create a remote control for the HU that you can execute from Android.
Let me mention another thing about the current state of my build. I have a Nexus 7 2013 whose stock kernel does not support OTG USB Host + Charging. You need OTG Host because the Arduino connects to the tablet via USB and you need to simultaneously keep the power flowing into the tablet since we won't have access to the tablet's charging port behind the dash. While there is a kernel for the N7-2013 called ElementalX by flar2 that does enable OTG USB Host + Charging, it's not the most stable thing in the world, and I can count on at least one random reboot per day. I can deal with that for a couple of months or so because the Timur rom that worked so well on the N7-2012 is being developed by Timur for the 2013 version. Here's some info on that: