I used to do truss rod replacements and neck rebuilds like that as part of my business. As the others are saying, it's a complicated job which often involves heat-straightening the neck and a new fingerboard and frets. Plus some repair of the finish. The cost will be $500 to $1000, depending on the skill and experience of the shop doing the work.
Is it worth doing? Well, sure, it will fix the bass and make it better than new. Is it worth it financially? What's the value of the bass?
Just so you know, in almost all cases where a truss rod is broken, the root cause is that the neck has developed a forward bow. Then someone broke the truss rod trying to bring it to straight. That's beyond what the truss rod is designed to do. So, the forward bow problem has to be fixed in addition to replacing the truss rod. Or the new truss rod will break too.