1 x Pololu DRV8835 Dual Motor Driver Shield for Arduino = @ $10.00 ea.
1 x Romi Chassis Kit - Pink = @ $40.00 ea.
1 x RedBoard - Programmed with Arduino = @ $31.00 ea.
1 x RedBot Sensor - Mechanical Bumper = @ $7.00 ea.

Depending on how much you want to thrash the motors, you might want a replacement regulator to power the RedBoard (knock off arduino) so you can use fewer batteries and run the whole thing at a lower voltage. Pololu 5V Step-Up/Step-Down Voltage Regulator S7V8F5 will give you 5v to feed in to the RedBoard's Vcc after the regulator.

Whatever sensors appeal to you, here are some suggestions:
1 x Mini Photocell = @ $1.00 ea.
1 x Ultrasonic Range Measurement Module - HC-SR04 = @ $7.00 ea.
1 x Light to Frequency Converter - TSL235R = @ $4.50 ea.
1 x SparkFun Sound Detector = @ $16.00 ea.

And maybe a servo to add an extra axis of movement:
1 x Power HD Micro Servo HD-1600A = @ $12.00 ea.