Upgrading your bass is a great way to get a huge improvement in sound and/or playability without spending the big bucks for the high-end model. If you’re really handy you can actually build your own bass, but most people will want to start with a decent bass and improve it.
With electric basses, the difference between the low-end models and the expensive ones is often the pickups. With modern equipment, it’s fairly inexpensive to turn out very nice bodies, necks, etc. and as long as they’re put together well, you’ve got the basis for a very nice bass.
Check out the models from the big name companies that are manufactured in Korea or Mexico, for example. You can often purchase a bass for a couple hundred bucks that is solidly built and just needs an upgrade of the electronics to be a really killer axe.
As an example, I have a Squier Affinity P-Bass, which is a copy of the Fender® Precision Bass®. The Squier is a nice bass, well built and solid, but the pickups are understandably low-end. Dropping a set of high-end pickups into it would be a major improvement for a couple hundred dollars and a couple hours time spent.
There are plenty of other tweaks you can do to upgrade the quality of a bass like this, but by far the best bang for the buck will be a set of high quality pickups. After that, you can add things like drop-tuners, better bridge, etc.
If you have basic soldering skills and can work a screwdriver, you can make these mods easily. You’ll find that there’s something very satisfying about playing a bass that you’ve upgraded yourself.
