I would suggest buying the parts online, or getting free samples. The parts found in a computer may be the same as required in the 50 dollar robot, but it would be a pain to find them and remove them. The parts that might show up in your computer would basically be capacitors , resistors, and the LED. The capacitors and resistors I would not recommend taking from the computer because their rating might be too small for your purposes.
The LED you could take ( the small light thing) ,google LED or light emitting diode for pictures.
I think that you should just try some parts from free samples. Personally, I have never received capacitors and resistors as free samples, (but then again , I haven't really looked hard enough for a provider) .
LEDs on the other hand I have received as free samples. Search the forum for more information on free samples.
HOWEVER ,DO NOT THROW AWAY THE COMPUTER
The computer can be a very good place to find other things like switches ,pin headers , motors, connectors, and platforms. By platforms, I mean using the computer tower case as a case for a robot or by using a cd drive case as a robot's body.
Don't get me wrong, there are other things that can be taken from a computer that may be useful. Do some more research on it if you require some other part for another robot.
But for the 50 dollar robot, buy or sample the parts,
Hope I helped,