| To do it right, it gets pretty involved. The top has to be removed and the bass bar moved to the other side. Your bridge and likely your fingerboard will have to replaced or at least reshaped.
I don't know what we're talking in terms of $$, but maybe some of the luthiers on the board would be willing to give you a rough estimate.
You might want to think about selling your current bass and buying one that is already set up for lefty. (all hail) Bob Gollihur, who is a member here, sells nice Bulgarian-made carved basses set up for southpaws.
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