The robot needs electronics skill, programming skill, mechanical construction skill, design skill. All these things are also expensive, do we have money to buy all the material. It can move to any direction and avoid obstacles. No, read on. You need an old tape recorder motor, two round printed circuit boards and four chargeable 1.5V batteries, a switch and some wire.
Basic idea is to install the motor to vertical direction and put rubber top on the motor pulley.
If you do not find a suitable rubber part, you can put shrink tube on the pulley. Three supporting rods a little shorter than motor pulley, so that your robot leans to on side. When the motor works it moves to direction where robot is leaning. When the robot bumps into a wall, the balance is moved to opposite direction and robot moves away from the wall. The most difficult part is make round PCB-boards.