Amp.
If you cannot (or get someone to) set up the action and intonate your bass then is not cheap, it is firewood. If you got a 2nd hand £60 cheapie P, set it up and dumped a second hand (£30 seems to be the going rate) SD quarter pounder into it then you are good to go. If you want a J or a 5 or 6 then you will be paying a bit more, but you get the idea. Tone REALLY comes from a decent power section in your amp. A nice pre-amp is nice, but with a weak power section it is useless. Even not-as-good speakers sound better with a decent power section. Of course we could all (and some of us have) spend £silly on basses, but a well set up cheapie through a decent rig will do the job fine and dandy.