Well, you need to think about what kind of events you want to have, and costs for things like pins/badges etc. I think you're looking at it from the wrong angle. Think like this:
If your pins cost $10 each
You want to have an even that will cost $100 dollars for the location
And $100 dollars per bus trip.
And you have 20 girls
You need 200 for badges,
And probably 300 for the event
Thus you need 500 overall
And dues should be 25 dollars for your first semester. (numbers are fictional)
You also REALLY need a finance major to do this stuff for you. Recruit one
Or have someone take a basic accounting class. Believe me, this stuff is 100% easier that way. Some other things to take into consideration are supplies for Greek Week/homecoming/Anything you want to participate in. Creating a savings of some sort. And money for your philanthropy.
Hope that helps a bit.