Then there is the time cost of wrangling kids in an out of them. My toddler can easily make it 15 minutes to buckle her in just on her own. A third would mean easily 5 minutes of to get everyone buckled in and only if they are cooperative.
The transportation costs are annoying, but worth it.
A lot of new parents haven't yet realized that a carseat is wider than the average adult. Meaning that cramped middle seat isn't getting a third seat without very careful consideration and the right vehicle.
It's coming to mainstream seats though. Graco has had the Slimfit3 LX out for a couple years now; we had 6 (3-across in a Toyota Corolla and Honda Fit) and they worked great except for the kids not really liking them. Britax has the Poplar and One4Life Slim; Chicco apparently added the Fit3x when I wasn't looking.
So, over here car seats may be a much bigger factor than daycare costs.
You could put two children through the local Montessori nursery for three grand a month, but frankly the parents (and children) there are a bit odd. Nice enough, just a bit odd.