As others have said the amount that you spend is much more about how much you want to spend than it is an essential cost.
I have a six month old baby. I have taken 12 months maternity leave and asked HR to average out my pay so that i get an equal(ish) amount each month (i get less in the months without SMP).
We are very very lucky. Most of my friends have finished having children and were ready to get rid of items like cots (including next to me and moses basket), bouncers, jumperoo, clothing, activity gyms, breastfeeding pillows, baby bath, toys, books, stroller, etc. Our initial outlay has been very low because of this. We spent £250 on a car seat that should last until he is four.
I got a lot of cloth nappies from freecycle, but we do use a disposable overnight, so spend about £3/£5 a month on nappies. We use cloth wipes, so no ongoing cost there either (bought for around £14 for 25 wipes).
He is breastfed, so again very little outlay. I got some bottles given to us for free, so spent £7 on a second hand manual pump and sterilising box so that we can express milk. In the last six months we have made it to the cinema four times.
I spent around £200 on an NCT antenatal class, which was worth it. I meet up with the women from the group most weeks. We take it in turns to go to each other house and hang out (which saves on baby groups/going out for coffee). We do go to a baby group together but started quite late so i have spent only £35 on baby groups in six months. We do go to some of the free groups in the library, and my son has a library card so we can borrow books for him.
We have just started baby led weaning (he eats what we eat) which has yet to impact on the cost of our shopping (but as others have said i expect it will lead to a healthier shop). We bought an ikea high chair for about £10 (new) and a splash mat for £15.
We did however spend £40 on buying him a kilt outfit (including wee sporran) to wear to a wedding.