Personally I think money isn't a fair factor in deciding on the number of kids you want to have. You never know what your circumstances will be in the future, things could and probably will change dramatically. Different work, more money, less money. Either way you will be able to make it work.
I used to work at the customer admissions of a zoo before the girls came along. There you get used to seeing different family sizes. But one day this bus came in, I went up thinking it must have been a school excursion, but no it was a family. Whilst I was processing the family through I remarked on the size of the family, something like wow, you have a larger family, 2adults and 6children. The father replied 'oh no dear this isnt the whole family, we have 6 more children at home!'
I was shocked and asked him how they got to do things like go to the zoo etc. He said it was easy, the eldest 6 got to go when they were younger, now its the 6 youngest turn. He went on to say that the eldest ones all had jobs and if they wanted to come on that trip with them, they just had to contribute to the costs. Sounded fair to me!
Anyhow my point is I think you would always make it work.
For us at this point in time, we are happy with our two, yes we would like one more. And from there who knows.
We have a series of factors for us, but it isnt based on money as such. At the moment I know we can handle two, We as a family arent ready for a 3rd just yet. I would love to return to work before another one came along, and right now we move aound a lot and I want to be settled before we welcome another bub into our home.