This really depends on what school you are at and how much they're charging for tuition + other stuff. You can look up specifics in your situation. Additionally, you can always take classes at your state school, in which case you'll be paying in-state tuition. If you're interested in that, you can look up stuff at UMass-Boston. I only mentioned HES, because they 100% have evening classes, meaning you'd be able to have a job in the daytime.
I'm currently a non-degree seeking student, and I have not paid for anything other than tuition for classes at my state school. My tuition is the exact same price that the degree-seeking students are paying (nothing more), and I don't have to pay for any student activities fees. Overall, the tuition per credit is about 2/3 the price it would have been had I taken them at my private alma mater. So no, in my experience it does not cost more per credit to be a non-degree seeking student.
You really should look up the specifics and compare cost.