Actually I am pretty sure Sprint is still making a nice profit off of me at only $50 a month, just not as big as their regular $79 a month customers for similar plans.
There are also plenty of other options like Pageplus, Straight Talk, Virgin Mobile, Ting, Boost Mobile and many others that offer plans from $25 to $60 for nearly unlimited everything and some allow an iPhone as well. They don't own their own towers or networks and just pay the big 4 carriers to use their networks. Even as middlemen companies they are far cheaper than the big 4 and everyone is making a profit presumably or they wouldn't be doing it. That tells me that the big 4 can make a profit for far less than what they charge and a lot of that cost is in the phone subsidy.
Yes, that definitely sounds sensible!
Still, I'm happy I can run my iPhone on 15€ a month, and have spent (iPhone comprised, latest model when bought) a global 1200 euros over the last three years... Pretty sure it beats most iPhone owners in the US :D