It goes with out saying that babies are expensive. I would like to suggest that you make an investment and purchase cloth diapers. It is an upfront expensense but it will save you money in the long run. You can save them and reuse them for your next baby as well. One thing you should know is hang drying your cloth diapers in the sun is the best way to kill germs! The sun is FREE and it does it's job well. When I was researching this topic a while back the experts suggest you have at least 24 diapers. Unless you want to wash each day :O)
One way to lower your cost is to look on craigslist and see if you can purchase the shells from someone done with diapers. Or you can ask for them at your baby shower.