I work full time (Monday-Friday days) for a hospital doing staff development and a few management projects. For a second job, I teach a course at a local university. While I enjoy my 2nd job, and I hope the experience will open doors for part-time employment down the road that will help me ease into retirement when I can no longer handle the stress of my full time one ... I am not making any money at my part time job. I also don't enjoy having to spend time most evenings and EVERY weekend working on my course. I am getting very burned out and longing for January, when I will actually be able to have my evenings and weekends free. I haven't had a weekend off since September and that is starting to be very stressful.
As for the money, I am making very little once you consider the many hours the job requires. Also, I will have to use vacation time to grade all the class papers in order to submit their grades on time. Once you have deducted the taxes ... then taken the value of the vacation time I have to use (which I could cash in for cash) into consideration ... there is not very much left that is actual "profit." But as I said, I am not doing it so much for the cash now as I am for the experience that may give more job opportunities later.
Have you considered picking up a few extra shifts with your regular employer? Sometimes that is a much better option to earn extra cash as you can sometimes get paid more per hour if they are above and beyond your normally scheduled shifts. Fewer extra hours worked, but more money sounds really good to me. You may also qualify for more vacation time, sick time, retirement matching, etc. by working a few extra shifts for your current employer. Don't forget that those benefits have monetary value that helps your overall financial status. Also, you get the advantage of being seen by the management as helping out by working those extra shifts. That positive reputation with your supervisors might bring better performance evals, promotions, positive recommendations for future jobs, etc. that could pay off big time later. In other words, you might get "more bang for your investment" that way. Whether that is true or not for you depends on the specifics of the situation.
Good luck with whatever you decide.