A much larger pool of online documentation, example code, and experienced hobbyists who like answering questions, is worth a great deal more than eighty cents.
Getting more use out of the battery is a good and sufficient reason to use a less powerful controller, though. It being cheaper is nice in theory, but almost meaningless for a one-off project, in practice. I can't remember the last time I made a purchase which was a fraction of a dollar, it just doesn't move the needle.
I haven't found a lack of documentation, examples, or available experts with most of the microcontrollers I use. It's great the RP2040 has those, but it's not a unique advantage to them.
The RP2040 does have a great advantage for people who are new to the microcontroller game, though! For me, personally, that doesn't matter much because I'm not new at this.
I agree, in a one-off or small run project, the cost of the microcontroller is usually irrelevant. I only mentioned it because when all else is equal, I'll go for the least expensive option anyway.
Getting more use out of the battery is a good and sufficient reason to use a less powerful controller, though. It being cheaper is nice in theory, but almost meaningless for a one-off project, in practice. I can't remember the last time I made a purchase which was a fraction of a dollar, it just doesn't move the needle.