Great news on the baby!
We saved a fortune on buying second hand stuff from e-bay, and then sold stuff back on there once they'd outgrown things. (prams, cots, etc)
I just bought my nephew a "bundle" of clothes, all Next & Gap, 100 items, £30-all like new.
You need to go to your usual supermarket (or look on-line) at how much all the baby sundries cost (nappies, wipes, baby shampoo, food, etc) and work out what he/she will need. Also formula milk if mum not breast-feeding. Then you will have an idea how much more to add to your housekeeping costs.
Oh and don't forget extra washing, soap powder, electric, tumble drying. As I recall, new babies tend to be beautiful, but fairly messy!
Good luck!
Blue