That's probably due to long standby times. You could get away with cheap (i.e., Harbor Freight batteries at $20/20 or comparable) if you were willing to change them more frequently.

I'm not sure what the actual scale of difference is between battery chemistries. Perhaps someone with more knowledge will reply and refute me.

You have enough smoke detectors that it requires 75 - 100 batteries to power them all?

The old Nest smoke detectors seem to use six of those batteries each. That is still a fairly high number of smoke detectors.

$200/$65.89 = 3 packs of 24 AA batteries

Nest smoke detectors take 6 batteries each https://support.google.com/googlenest/answer/9297548?hl=en#:....

72 batteries / 6 batteries/device = approximately 12 smoke detectors (could be 9, 10, or 11)

We have 6 First Alert smoke detectors that are connected together but don't use WiFi. There's 1 in each bedroom and a couple left over for key places.

Using 12 smoke detectors is being very careful to cover every space or a big house. Or both.

I just counted, I have 10. I thought it was 11, and I rounded up. Anyway, the exact math wasn't my point, but that lithium primary batteries are very expensive. Ironically, I just got a price alert that they're down to $46.64, except that when I try to buy them, Amazon says "Sorry, this item is no longer available from the seller you selected".

I don't have a huge house, but it is a 1950s split-level, and is pretty much the opposite of a modern open-plan design. And yes, I'm careful. Following NFPA recommendations, I have a smoke/CO detector in every bedroom, outside every sleeping area, and on every level, which adds up. Better safe than sorry.

How long do they last? Normal lithium ion powered smoked detectors will last 10 years between battery changes.

I think they claim 5 years. I don't keep track of this stuff very carefully but based on how long I've had them and being on my second round of battery replacements, I'd say I got more like 3-4 years. It's not that big a deal in the grand scheme of things, especially when the batteries were cheaper. I just got sticker shock last time I went to buy a set.

Until recently at least, Energizer was the sole supplier of these batteries. You can't use anything else because they have a different voltage and discharge curve, so if you put alkaline batteries in, the Nest will almost immediately complain that they're low. I see there are some mildly sketchy other brands on Amazon now. Not sure if I'm going to take that chance.

That's not lithium ion batteries. That's lithium batteries which are not rechargeable so you have to buy new ones when they run out of charge.