Self-Balancing robot project starts!

I just had an idea yesterday that can get me to make a self-balancing robot without buying any expensive electronic components.
You know last summer i made a cellphone controlled RC Car which had a tabletpc onboard to get commands via bluetooth.That time , having an onboard tabletpc , save me from buying a bluetooth chip which costs about 60$.This time, to make a self-balancing robot i’ll have to have whether an Accelerometer or a MEMS Gyro to track rotation of the robot.
Those sensors are pretty expensive too.So i’m thinkin about putting my n95 smartphone onto the robot.As i have Accelerometer on the smartphone i can use it to measure rotation movement of the robot.

I tried to explain how it is going to work in the photo.

And the finished robot will look something like this: