Well, for me I have to spend 10-15K per attempt to conceive. That's a whole lot to try to afford. I'll probably get the bills from this pregnancy paid off shortly before dd arrives, assuming she can wait until her due date. The only reason we decided to have another child so shortly after having ds (he'll be 2 next week), is because dh plans on quitting his job next year and going back to school, so bye-bye health insurance. We don't plan on having anymore kids until he's out of school and we're stable, which will probably be 6 years from now. It takes a whole lot of money for me to get pg, and I have difficult pregnancies which land me in the hospital for preterm labor (more $$$), and unfortunately I also have complicated births (even more $$$). With health insurance, it's doable, but without it, I'd seriously be worried about losing my house due to medical bills.
We don't take vacations. We spend money on food, because I think buying natural, organic foods as much as possible is important. We live comfortably, but not lavishly. The expense of another child once s/he got here wouldn't be an issue, but the expense of getting him/her here could break the bank.
Emotionally, I don't think I could afford another child until my ds and dd are old enough to be at least partly self-sufficient. I've had to farm out a lot of ds's care to family members, because of my difficult pregnancy. That really makes me feel bad as a mom.
I think that people have to make their own decisions about what they can and can't afford. To some people, their yearly family vacation may be what keeps them sane and makes them better parents. Some people don't want to stress about having a car that will hold enough carseats. Some people would probably go crazy with a small house full of kids. We're all different, and we all have different threshholds of what we can take financially, emotionally, and spacially.