Going rates for a nanny are around $25/hour, which comes to $50K/year for a standard 2000-hour work-year.

And yes, nanny-shares are considered a "budget" option in the Bay Area, for when you can't afford a good preschool or can't get off the waitlist. Unfortunately the quality tends to be more variable...I've heard some horror stories about irresponsible nannies. There isn't a vetting & licensing process the way that there is for institutional childcare.

If you only have one, $4k/mo is still cheaper than a nanny in MTV especially if you don't have an extra bedroom to subsidize with room/board.