Are you planning to breast feed or use formula......even then that is hard to predict because sometimes the whole breastfeeding thing doesn't work out.
I would say roughly $60 for diapers, $125 for formula (depending on which brand you use) and say about $150 for miscellaneous items. (like baby wipes, shampoo, clothes, baby food) Keep in mind that these are just estimates, some months it may be less and some months it could be little more. Also are you going to still have to work because then you have to look at the cost of daycare.
Hopefully your friends and family will have a baby shower for you and you will get a lot of your stuff then. Many people like to give diapers, wipes and shampoo ect....