First, I have to sympathize with your situation, as I was once in a similar predicament. My parents did indeed cut off their finacial support, and I did join up (the Navy, enlisted). I can tell you that my parents' initial response was shock and horror. On the other hand, once my decision had a chance to sink in, they got used to it and they very quickly came to understand what an excellent occupation the military can be. My parents couldn't be prouder of what I do now (even more so since I made the switch to officer, and pilot training, but they were justifiably proud of my enlisted service as well).
My parents issues with the service I think sprung from my mother's father who enlisted in the Army in WWII and was made promises by the government that were not honored. Along with those concerns, both of my parents grew up during the Vietnam war and (I believe) still harbored some of their liberal (hippie, commie) misgivings about that war. Seeing how the military actually is has really brought them around. I mention this just in case your parents fall into the same catagory ... if not, disregard.
Best advice I can give you on their financial concerns is to look at what a Marine officer actually makes per year, then look at your prospects out of college. I know that in my field (Chemistry) it was possible to gather up the trade journals (the ACS magazine in particular) and find out the average salaries for new graduates. The Marine Corps Times does an issue each year in which they estimate the actual equivalent income of service members and compare them to their civilian counterparts. In my opinion they significantly inflate the value of some of our benefits, which should work to your purpose.
Hope that this helps a little ... good luck!
