I have a good job and salary now. And I want to quit my job to take a Ph.D. degree, which means my income will be cut in half. Should I mention that in my personal statement to prove my dedication to academia? Would that be like I am looking down the academia as I quantify the benefits academia brings me with the money?
In my opinion no. You should mention your current work and any relation it has to doing a PhD, and that you wish to now do research and gain a PhD. Emphasize why you want to do a PhD but do not compare your current job with that. Anyone reading your reference will likely make positive inferences about you having real-world experience and that you are following your heart. Generally try and steer away from any negative comments.
Being honest about your intentions should never be a bad thing...unless of course your intentions are bad! - I would suggest that you state that "in spite of a good job and good salary..." (or some such words) that you feel compelled to pursue your PhD (for whatever reasons you have.) Quantifying the difference between current salary and PhD stipend does not necessarily 'prove' your dedication to academia so I would leave it out. Good luck!
If you don't mention it, they may have concerns about your plans. Broadly speaking, the "why do you want a PhD" discussion is much more important for applicants like you than it is for straight-from-college applicants. Some concerns they might have:
- Does this person realize how low the stipend is?
- Are they really prepared to give up that much money just for a PhD?
- Are they going to try to negotiate for part-time status or something else that we don't allow?
- Are they going to drop out after a year because they can't survive on the stipend?
- Do they have a realistic understanding of what we do here? (As opposed to assuming that academia is a magical place where everything is perfect except money.)
- Do they have realistic plans for what to do after they graduate?
So, I think giving some indication that you are aware of the financial implications of your application and have a plan for approaching them is very appropriate.