For us, frugality was not a factor in planning our family. But, our family planning dynamics went a little differently than "planned." When I met Dh I already had DD1, and she was 6 when DH and I got married. We started trying right away for another, but ended up dealing with unexplained secondary IF. We didn't get pg with DD2 for 6 years, and that was after an IVF (which insurance paid for thank goodness.) There are 13 years difference between DD1 and DD2. After DD2 was born, we started saving for another IVF, but then I ended up pregnant again, all on our own, when DD2 was just 13 months old. Once DD3 was born, we weren't sure what we were going to do, and I ended up pregnant again, DD3 was only like 15 months old. We decided we were DONE, but finances was at the bottom of the list of why. DH's age, my age, how close the kids were, the fact that I was tired of having babies, being pg, etc, all that was more important.
Having said that...having 4 kids is EXPENSIVE...even if 1 of my 4 is only here part time now that she is in college. If I want to swing through McDonalds for a coke, it ends up costing between $4 and $5 after taxes, because I have to get a drink for everyone. Christmas can get crazy with getting even a small quantity of gifts for each. I could easily spend $500+ on JUST my kids, that doesn't even count getting a gift for DH, or any of the rest of my large family. Because of that, if age and closeness of the kids hadn't been a factor, costs would have been a major one.