This, what Dave said.
What should be remembered here is the difference you get between playing live, and recording. You could upgrade pickups with some inexpensive Wilkinson or Entwistle's which will improve the tone of a cheap bass in a live situation, and as well for recording. However, upgrading to high end pickups won't be as big a difference as the Wilko's etc, but for recording, the high end pickup will make a huge difference - even over the Wilkinson or Entwistle's.
Also, tone is a subjective thing.