I'm shooting from the hip here, so be forewarned. I think "besides" made be shortened from something. Maybe "besides the point." Anyway, here the first reason for whatever the speaker is about to do is because she needs the money. "Besides" kind of pushes off the table so to speak. She agreed to do it, so she would have done it regardless. So, besides it kind of like regardless, but here I believe the context is more of a rationalization. That's to say, she doesn't really want to do 'it', but she needs the money and she agreed to do it.