I agree with PPs that it isn't the baby year/s that are the issue... it's very definitely the older kids! It's already starting with DD, and she's not even three yet. The hand-me-downs have tapered off... toddlers are rougher on clothes than babies, so while everyone seems to get piles of hand-me-downs for newborns, it gets harder once they hit toddler age. (Plus, a lot more people sell their toddler clothes online these days - still cheaper than buying new, but not that much cheaper IME).
And at some point they start needing shoes. DD's last pair that DIDN'T hurt her feet were $80, and she'd thoroughly trashed them by the time they got too small - no way we could use them for kid #2. She's never done classes, preschool, daycare etc yet, and I do plan to homeschool, but it turns out she (miraculously!) has an uncanny ability to dance, so I'm going to put her in a toddler dance class as soon as I get my driver's licence. Who knows how much that'll cost - but I really feel that I'd be doing her a disservice by ignoring her talent and interest in dancing.
And I'd like all my kids to learn an instrument, unless they absolutely hate it - that costs money.
DD's also just started to eat enough that I notice the food missing, too. For a VERY long time she ate little scraps off my plate, but all of a sudden we're buying avocadoes because she loves them, cooking three eggs instead of two, buying an extra chicken breast... it adds up. Obviously I don't begrudge my daughter food! :p But she'll only get hungrier from here on in. She had her first "individual" ice cream the other day, because I knew she'd drink half my milkshake if she didn't get her own.
Also, don't forget that even pregnancy can cost money. In NZ most medical stuff is free, but I've still shelled out for a fair few supplements. And our takeout bill (partly due to my cravings, but mostly on days I've been too sick to cook) has gone up rather a lot! I haven't been able to keep up with breadmaking, so we started buying bread - for the first time since we got married! *sniff* Little things like that... and if you WAH, you might need to cut back on that if you're sick...
Obviously, there are lifestyle choices you can make that makes having kids cheaper. But everyone has their areas of non-negotiability, and some are more expensive than others. I REALLY don't want to be in a financial position where I have to cook my kids carb-heavy fill-'em-up meals, like I ate when we were growing up (big, poor family). I believe my health suffered from eating rice and pasta for years because meat was too expensive. So for me, a semi-TF diet is pretty non-negotiable. For other people, it might be family holidays or private school or cosmetic dentistry or paying for their kids' college education, and I don't think it's fair to say any of those choices are "wrong" or "unnecessary" or "materialistic". Different people find different things important.
Which I don't say to put you off having kids. :p But yeah - it's simple math. Two kids will cost more than one. You can't get around it. Unless you send one out to forage for his meals, and make them shower together until they leave home, and so on...