i'll go against the collective on this. the easiest way to get a big change from a bass is to change the pickups. it's not always a better or worse, but they certainly don't all sound the same just because they are made of the same stuff.
Yeah, I was a bit surprised reading here that people don't think pickups make a big difference. Excuse me? Pickups are *the* difference (well, and strings like you said). Sure, with EQ you can make one pickup sound like another, but how many of us play with a graphic EQ on stage? If a pickup has a weird sound that you can't improve with whatever preamp you're using, then its a bad pickup for you. Not saying its a bad pickup, but a bad combination for whatever you are trying to accomplish sonically. With unlimited gear almost anything can be done, but that's just not the case for most of us. I choose pickups that sound great with my rig with the least amount of knob turning.
I do ok. Just stating I know a lot of musicians and only a few who would know blind.
Blindly being able to pick out one from another is one thing, but it doesn't really matter. I've owned a Fralin stock wound p-bass pickup, an Aguilar P, and a Duncan Antiquity II. If you took the three of them, popped them in the same bass, recorded them then played me the samples, I guarantee you that I wouldn't be able to tell you which one was which with any greater success than I would have flipping a coin. But I also guarantee you that I would hear a difference between the three.
At the end of the day, your playing style probably has the greatest impact on anything, but if you're going to have multiple basses, I would rather have ones that let me achieve the sound I want without having to mess around too much and fight against the tone of the bass to get a specific sound, and pickups play a huge roll there.
As to expensive vs cheap, I haven't seen any night and day correlation. I've played expensive pickups that didn't do it for me, and cheap pickups I loved. Where its worth spending more money is customer service. I was happy to pay a higher price for a couple Fralin pickups because I was able to pick up the phone, ask for Lindy, describe to him the tone I was after, the amp I was using, the play style I had, and he wound me a pickup that absolutely nailed what I was after. Plus if I was unhappy with it or wanted a different sound he would rewind it for me. To me, that piece of mind was 100% worth spending money on.
Years later I switched over to an Aguilar set of pickups and you know what? For the time and place I was in, those off the shelf pickups were perfect for what I was doing and I got rid of the Fralin.
Later I played a GFS p-bass pickup in a buddies bass that I thought was perfect for a specific tone I was after, but for one reason or another I never ordered a set. But I'm pretty sure I could have happily replaced my Aggys with the GFS, for a while at least.