Same for $400 or even $300.
My guess is $100 to $120 would be the top limit for most consumers. For me it'd be under $100.
How much would readers be willing to pay monthly for mobile data? (Feel free to answer below.)
And where does that leave companies like AT&T and Verizon once the vast majority of their customers own smartphones and have limits to how much they'll spend on commodity mobile data?
Maybe they'll need to offer unique, value-added content. That's why I wouldn't be surprised to see either AT&T or Verizon buy Netflix (AT&T's recent battle with the streaming-video company over a bandwidth cap for DSL customers notwithstanding).
If you're just offering a service, and people are only willing to spend so much for that service, you've got to sell them something else if you want to grow revenue. Otherwise you (and your stock price) just stagnate.