My baby hasn't really cost us much. My 7 yo costs us loads.
The baby is 1 yo and has cloth nappies and wipes, was bf for 10 mo, so there were formula costs for a bit but I think only about 3-4 tins of it. The clothes get handed down and it's just all babygros really anyway, they are 50p each in the local charity shop, clothing a baby is very easy to do. We haven't needed childcare as I can take the baby to work with me. We have a hand me down cot and don't use a pram / stroller. There was the car seat, that was £60 in the sale but it's a first class convertible so lots of time left with it. Books are buy one get one free at the charity shops in town all the time and pennies to begin with - even if I want something specific I can usually find it for postage online. The baby can't walk yet (13 mo) so no coats or shoes or outdoor stuff needed yet. We spent £8 for the baby's first Christmas and £10 for the baby's first birthday. I don't use puree or special baby foods, I am happy for the baby to eat what we eat from day one so no special foods costs or spoons, blenders etc. I did buy a couple of the cheapy tommee tippee beakers when I started giving some formula. I was given a breastpump. I don't have a changing bag or changing mat or a baby bath, but I was given a baby bath seat which was good, the baby uses normal towels. I can't think of much else really.
The 7yo, though - swimming lessons, sport, music - wraparound care - holiday clubs - school clubs - school dinners - uniform - shoes!!! Food, snacks (always, always hungry) days out, special events, birthday parties, pocket money, bus and train fares, entry into places - and it's only going to get worse I'm sure.
Yes, babies are cheap - I'd say ours start getting spendy when they start school.