How can I enter into the mathematics field at the graduate level when I only took a few math courses as an undergrad? Background
I was ignorant as an undergraduate and didn't focus heavily in any specific area. I finally graduated with a single math/computer science degree that consisted of 5 math classes beyond multivariable calculus and a few relatively basic computer science classes (algorithm analysis, SQL database design, etc.). My only research experience is in economics.
Now, I work at a research institution in economics where I primarily write Stata/MATLAB code.
Question
I'd like to get more mathematics under my belt over the next few years in order to either enroll in a PhD program in economics/finance or a M.S. program in finance. What are my options for getting more math experience that I can use in graduate school admissions?
Options
These are the options I thought of so far, but I would like to know more if possible.

Study enough courses through a system like Open CourseWare to gain enough experience to enroll in more advanced undergraduate classes part time, which my employer might pay for, and then ideally move into an M.S. program in applied math.

Return to my alma mater for a M.S. degree in math that would at least give me a basis to either move into another math graduate program with higher rankings or a lower-ranked graduate program in the fields of my choice.

Join the Air Force and hopefully take advantage of the GI bill to take more courses somewhere and gain entrance into an M.S. program in applied math.

(Sadly) Forgo advanced mathematics and find a consulting job, which although unpleasant in the field of economics (in my opinion), wouldn't be difficult to obtain given my research background in the field.


Do I have any other options? I also plan to take the GRE subject test in mathematics to prove that I have at least minimal knowledge, regardless of what course I pursue (since many graduate programs will require it).
It sounds like your best bet would be to contact the graduate officers at the economics and finance programs you're interested in attending, and ask them what their minimum entrance requirements are. It's a much more efficient route than just deciding to take more math courses, and spending significant time, energy, and money on a quixotic goal.