It completely depends on:

• The kind of relationship you have with your advisor/supervisor and
• The extent to which s/he has gone out of their way to help you over the course of your Ph.D.

In most cases I would say no, because you are the underpaid employee and s/he is sort-of the boss. Would you give your supervisor at work a gift after finishing a significant project? Probably not.

PS - It's true that it's not your advisor who decides that Ph.D. salaries be low, but then, that's usually true for your supervisor at a regular job.

It completely depends on:

• The kind of relationship you have with your advisor/supervisor and
• The extent to which s/he has gone out of their way to help you over the course of your Ph.D.

In most cases I would say no, because you are the underpaid employee and s/he is sort-of the boss, or at least the representative of the university. Would you give your supervisor at work a gift after finishing a significant project? Probably not.

PS - It's true that it's not your advisor who decides that Ph.D. salaries be low, but then, that's usually true for your supervisor at a regular job.