I know that Cox has some kind of "low speed, low price" offering, that I believe is under $30/month - but they kinda hide it on their site; I think you have to explicitly call and ask for it. See here (scroll near the bottom):
https://www.cox.com/aboutus/policies/speeds-and-data-plans.h...
Note the "Starter Package", then note the package one level up ("Essential"). If you click on the link next to the "Starter Package" for pricing, you will be taken to another page - where the lowest shown tier is for the "Essential Package". But I'm pretty sure you can get the "Starter Package" if you call first.
There's also this for low-income families:
The only way for that ARPU to drop considerably is if a new player enters the market that can significantly undercut the existing players, because none of the existing players are willing to see that number drop by half or more. I don't believe such a new entrant will materialize anytime soon due to the high barriers to entry. I suspect the cost of building and operating a wireless network in the US is too high to support a carrier operating on a fraction of the ARPU.