Nappies (huggies) I buy when on special, safeway often has 2 boxes for $66, sometimes you can even buy box for $30.
Formula a tin prolly lasted me roughly 6-7 days, cost can vary on the brand you use. Don't buy at the supermarket, way expensive, go to chemist warehouse or pulse pharmacy is quite cheaper too.
If you're regularly putting money away fortnight thats a good start, maybe even try one of those higher interest savings account, I had one thru bankwest, got good rates and didn't have to lock it in could touch the money when i wanted.