I used to get milk powder from JB Tesco, not sure of that's the cheapest though but I stopped after I found a Chinese medical hall near my place selling it cheap. If you get from JB, a 900gm Friso costs about $20-21 sing dollar after conversion , I'm now paying $82 for 2 cans 1.8 kg Friso 4, the infant stage one of course will be slightly more expensive but I thought about the same so save the hassle of going in JB. The Chinese medical halls do sells diapers as well and its way cheaper then getting it from Ntuc etc.
But if buy from JB then can pump petrol also if you drive in.
If you are really struggling in terms of budget, for breast pump and stroller, can buy second hand one.a lot of mummies selling in this forum. Baby clothes if you dont't mind can get second hand ones also since they outgrow them so fast. Plus relatives and friends will give during full month .
Hope this helps