Feed costs vary hugely depending on where you live...I've got the same size flock as chickens really, and I definitely don't spend $100 monthly on feed, more like $30 to $40.
Tiggirl, it's great you're doing your research, but don't be overwhelmed! Chickens really are easy once you have them. I've bought all my chicks and ducklings from a local feed store. I just looked at them closely and made sure they looked perky and happy and were lively. Like other animals...