TPS7A20 is 6.5uA, $0.10/ku. TPS7A05 is 1uA, $0.19/ku. It's not that hard to do better than 400uA!

And if you don't mind coughing up when it counts... TPS7A02 is 25nA, $0.45/ku. Triple the price, but more than four orders of magnitude better leakage.

If money is no object, there are some interesting chips out there. The LTC3335 chip[0] is a buck-boost that claims to have a 680nA Iq. Not as low as the TPS7A02, but it looks like it could make up for that in efficiency if your design does anything other than sleep.

[0]: https://www.analog.com/media/en/technical-documentation/data...

I was looking for one for one of my project but this is above "I could have just 10 years worth of batteries instead of that" level of pricing