I have 3dc. It is harder having 3 for several reasons. Some examples of things that have become much more difficult now we have 3 children:
Going swimming - it is hard taking two children by yourself, but virtually impossible to do safely with three small children.
Going to the park/playground. - watching two children is ok, you can position yourself so that both are in line of site, but with three, it is inevitable that one goes out of line of site when they all go off in different directions. It was only when the third child was mobile that I found that out.
Hotels and package holidays - all cater for two adults two children. Fine when the smallest is a baby, you can use a cot, but then once they are out of a cot, the cost of going to a hotel is extreme as you need to book two rooms rather than getting away with one. We never did the holiday thing before, but occasionally do stay in a cheap hotel when on-route somewhere or at a wedding for example.
food Packets - (eg yoghurts etc) they come in 4's often.... (hence one reason I have decided to go for dc4!)
more childcare difficulties. Finding someone to look after two was never an issue, finding someone for 3 for babysitting overnight, not so easy.
childcare expenses if going back to work.
One big one..... CAR SEATS in car, many cars do not fit 3 car seats across the back. Many many people caught having to buy new smaller carseats or a bigger car.
OK so having given loads of reasons why not.... Yes I would never be without my mumber 3, he is fantastic, and now we are hoping for a number 4 child.
Really, it is a tough one. Every couple has to work out the best for them. I would never put money directly as a reason to not have another child, but there are plenty of other reasons.
I hope you work out what is best for you!