With nappies, it'll cost around $1000 to set up in cloth (MCN) nappies, and I'm not sure about sposies. Count on about 6-8 nappy changes / day to begin with and a box of 100 huggies is around $50 (ish). If you're planning on using sposies, then stock up now - toys'r'us have good specials, and most shops let you exchange for larger sizes if the box is unopened.

Even if you're planning on BFing, plan for FF. 2 tins of formula @ $30/week until they wean (roughly) and if you want to express, then hire a pump (better quality & cheaper).
Either way, you'll need a steriliser and some bottles.

Clothes are a big "gift" item as are bunny rugs and toys.