I voted environmental because that was my first and last thought when making the decision. Then I went and did some math (I'm compulsive like that). So far our stash has cost ~$1,000. That feels like so much money. Some of it came from our baby shower or gift certificates so I don't feel too bad. We also plan to sell some of the Swaddlebees because ended up with too many and don't get through them before we need to do laundry anyway. So a little bit of that will come back. We will be buying more in the next couple sizes, but not that many because we are already starting EC and we're doing really well.
My guesstimate is that we will actually spend somewhere in the neighborhood of $1,500 by the time we are done with diapers. (I like pockets a fair bit.) I may get this amount lowered by asking family to buy diapers in the next sizes for Christmas.
If I spent $15/week on disposable diapers (which seems to be about what it would cost based on my research) I would be spending $780/year. We are planning to have three kids. This means that I will have made my money back on a little over three years of diapering babies and I will probably have somewhere between 6 and 8 years of diapering babies. w00t.