That depends.
Not all basses are equal. Some plys are better instruments than some carved basses.
Financially, the point of diminishing returns hits when you experience a bad bass bar crack. The cost of repair can exceed the value of a cheap carved or hybrid bass.
Sonically, you have to lard up a decent carved top with a lot of weight for it to sound worse than a mediocre ply top.
The question frames a false dichotomy. Find a good bass. Play the heck out of it. Care for it appropriately. Have a talented luthier on speed dial when it inevitably needs repair.