There's a huge compounding economics of scale effect for kids. Not only do consumables become cheaper in bulk but there's more opportunities to reuse. Three boys two years apart can all ride in the same car seats, wear the same clothes, ride the same bikes and use the same baseball gloves. I recall my brother (7yr younger) wearing shirts I wore in grade school after getting them from cousins that weren't the same cousins we gave them to. If the time between children is very little or they're not all the same gender (preferably boys, hand me down everything doesn't fly with girls for the most part) the "unit cost" goes up.
In a lot of middle class families teenage sons wind up learning skills that their fathers don't have so the net loss can go down in those years.
The way financial aid for college "works" (quotes indicate sarcasm) is you basically get told what you can pay and schools make at least a token effort (depending on the marketability of your kid, e.g black jewish straight-A D1 athlete majoring in something trendy gets more aid than an average white dude getting average grades in an average major) to try and not exceed that. If you can pay X and have Y kids in school then supposedly you pay X/Y for each kid in college. After adjusting for reality it's probably north of X/(Y+something) but the point is that it's cheaper than if you had two kids 4+yr apart and are only ever paying tuition for one at a time.