Depends on the person and the track that you run on. Some people buy the expensive dif and love it on a certain track and on others they run regulars. If you feel like spending that much on a differencial go for it but you could save for another motor, new servos, new radio, new pipe or many other things.

The standard setup for gear difs in a 1/8 scale buggy is 5k front, 7k middle and 1k rear.

Steve