Like tomearp said, your local dues fund your operating budget.
My chapter, as an example, has 2 budgets: operating & program. Our program budget goes to fund things like our Delta Academy, Health Fair, HIV/AIDS Conference...any activity that falls under our 5 Point Program Thrusts and our 2 Comissions. The operating budet goes to handle everything else. To budget appropriately, you need to sit down and figure out, in as much detail as possible, what you're going to do as far as programs and socials and what you'll need to pull those activities off. And don't forget to budget for office supplies.
Fundraising is another thing you need to consider as a suppliment to your operating and/or program budget. Plan your fundraisers and their timing carefully. You want to maximize your fundraising potential in two areas: members participation and dollars raised. Ideally, you don't want to have a fundraiser every month. It doesn't look good if everytime the student body turns around you're asking them for money. If you have any business majors or folks who just like to do this kind of stuff, defer to them as subject matter experts. There's also a
Fundraising Fourm on GC where you can get ideas.