I have 3 dcs ds1 (7) ds2 (5) and dd who is just 7 months.
DH was resistant initially to having a 3rd mainly for the practical reasons already discussed. Then one night we had a heartfelt discussion about it and I told him that our family just didn't feel complete to me, I still had a lot of love to give and I didn't just want another 'baby', I wanted another child to complete our family. I also pointed out that at my age, we didn't have the luxury of time on our side and if we were going to have another it had to be then. He then fessed up that he would really love another but was worried about the cost, car etc and it was holding him back.
We talked for a long time before deciding to go ahead, try and see what happened - I conceived that month!
We have never been flush, but we get by. Was a bit of a shock when I found out I was having a girl, as I was hoping to use a lot of the dss clothes etc but to be honest I have done that anyway as their sleepsuits etc were all white and I team their little dungarees with pretty tops for dd. I have also bought most of her clothes of ebay (most of which were bnwt) at very little cost. I have only bought her two toys since she was born as she has all her brothers old baby toys.
We did have to upgrade our little Toyota coupe for a second hand Citroen Picasso though.
It is true that a lot of visitor attractions etc tend to charge for families as 4 people rather than five and obviously as she grows food bills and treats for the dcs will be more expensive, but we will just have to make sure that we make the most of the days out and treats we can afford.
AND dd is a treasure, she has slotted right in, her brothers both adore her and she is absolutely in love with the pair of them. Personally, I found it easier going from 2 to 3 than from 1 to 2, but I guess it was easier for me, as the boys are at school all day during term time, so I had plenty of time alone with dd in the early days. That said, the summer holidays haven't proved a problem either, although I will admit to being nervous beforehand about coping with all three every day for six weeks while dh was at work.
I think, at the end of the day, life is what you make it. As someone else said (sorry I've forgotten who it was) if the love is there the rest will come.
My family was on a tight budget when I was growing up, in fact we were considerably poorer than all of my friends families. It never bothered me for a second at the time and the things I remember most fondly from growing up are the things that cost nothing at all like playing with my sisters, going on a picnic, cuddling/reading/baking with Mum etc
Sorry - all a bit rambly really, but hope it helps.