Therefore I decided to design and build one with the same functionality but lower cost.
Let's see how much it would cost approximately (I got all the prices from ebay):
Arduino UNO - £15
Servo - £3 (usually sold in bulk)
IR distance detector - £2 (will only need 2 or 3 units, I bought 10 from China for £3 ;-p )
Motor Driver (L293D) - £2 (cheaper than building one yourself!)
Gear Motor - £13 (better than using the motor directly as we need higher ratio, slower movement)
9V PP3 battary - £3 (I use rechargeable so more expensive)
resistor/capacitor/wire - £2
other accessories - £5 (varies really)
Total ----- £40 to £52 (which is $62 to $80 on today's exchange rate!)
I have divided the robot into several parts, and we will be building and testing each part before we put them together. The green arrow indicates the interactions between these parts.