A good nanny for four kids is definitely more than $6k / month in lots of parts of America, you're ignoring the downsides of nannies (like when they get sick, or need time off, etc), you're ignoring a lot of the upsides of daycare (like socialization), and the research does not support your claim that kids do better being raised by a nanny than daycare.

It's totally fine that you've chosen a nanny over daycare. I did for my first, and I think we'll go with a nanny for my second, but you're presenting a wildly misleading perspective here.