Later edit: 14.03.2017
The Balboa 32U4 Balancing is a robotic kit that can be programmed with Arduino IDE and the Arduino libraries.
If you want more, the ATmega32U4 controller has an interface for connecting a Raspberry Pi board. If you connect a Raspberry Pi board to the kit, you can control it via the Internet or Bluetooth.
The price is $69.95. The two wheels and DC motors are not included, which is not necessarily a surprise. This practice is quite frequently and many manufacturers sell incomplete kits.
You can’t use the kit without wheels and DC motors. So, Pololu sells wheels with the same size, but colored in five different colors.
For DC motors, you choose between three different variants, each of it with different gear.
At the price of the kit, you have to add the price of the two wheels
(2 X $9.25) ($9.25 for a pair), and the price for two DC motors (2 X $18.95).
The kit comes disassembled and requires soldering. During assembly, you need a soldering iron,
solder paste, and solder wire.
The kit includes an IMU sensor (accelerometer, gyroscope, and magnetometer), a 5V regulator able to provide a current of 2A, and two H-bridge motor drivers.