I agree that it's definitely better to do it now. IF there is ANY way that your sisters can help you work through this situation, they'll have more time to figure it out now than they will at the end of the semester (with formals, finals, graduation plans, etc.). Springing it on them right before you'd be completely done with the sorority will probably make them think that you didn't much care to try to work out the problems you're having (even if you tried everything you could to stay). It just won't look good to drop on the last day that anything can be done about it... even if there's no possible way to fix it.
Also, I don't know what your specific financial situation is, but with the way the economy is right now, I would like to think that chapters would be much more understanding when someone indicates that they might not be able to afford their dues.
Talk with your big. And definitely find out the exact amount of your dues. That shouldn't be something that you have to question and wonder about.
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I believe in the values of friendship and fidelity to purpose
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