Here's my 2 cents after reading every post on this thread. (is there a reward for this?)
My first bass when I first started playing in high school was a cheap Ibanez copy of a gibson EB-1.
I almost fell over when I handed it to the more experienced player in our jazz band and he completely ripped it up on it.
How did he do that I remember thinking?
I was also studying stand up bass at the time, using a beginners bass, and was making pretty good progress and I was on the Ibanez too, but when trying to play fast, advanced lines, I could never get it to happen on the Ibanez.
After getting interested in funk, fusion, progressive rock, tapping, etc, I started playing every bass that I could get my hands on in every music store I could get to.
The main difference between my Ibanez and the better basses was build quality and the ability to play without as much force in my fingers.
I wound up selling off everything I owned and cleared out my bank account to buy a used Alembic Series One.
At that point everything changed for me technique-wise.
After a while, (a long while), I played without as much finger pressure, and could play faster, more expressively, and could slap, tap, and play with floating thumb technique.
The "better" instrument helped get me there, just as professional string players look for the instrument that allows them full expression and to maximize their technique.
The kicker here is that now that I have a technique, I can pick up a more inexpensive instrument and still play it like I want to because the technique is in my fingers more than it's on the instrument.
In fact I picked up a used, 24 fret, SDGR Ibanez at guitar center for $99, that someone swapped out the active pickups for passive ones, and the bass has GREAT action.
I can rip on it as I can with my Alembic and tap and everything else.
That said, I'm glad that I played the Alembic in my formative years as I believe, from personal experience, that a well built instrument that you practice on seriously can get you to play better.
Now it also certainly depends on the kind of music you want to make.
But if I'm going to be playing something technically demanding from both an endurance, finger stretch, and speed standpoint, I'll take the better built instrument every time.
I had a chat with the guys at Westwood music once who had set up a bass for Jaco and the strings were crazy high, but right for Jaco.
I bought a standup from a very accomplished bassist who told me that I played it better than he did, because it was a better fit for me.
He was, of course, the better player.
IMHO the ultimate answer to the OP's question is, it's not the instrument that's more expensive, it's the instrument that's best built for you that allows you to play your best and maximize your technique and continue to build on it.
Just my 2 cents...