If you have an unlocked phone or can get Apple to unlock it somehow (I don't know exactly how this works) you can possible take it to one of the MVNO carriers like Boost, MetroPCS, Cricket etc. They offer some plans for unlimited everything for around $50/mo. They require GSM devices though as far as I know.
cricket = AT&T (gsm after take over)
metro = tmo (gsm)
boost and virgin are sprint prepaid carriers and require sprint phones
i heard they won't even activate used phones on sprint and you need to buy an actual boost phone
the walmart $30 plan is great. you even get unlimited music streaming with it
music streaming isn't available on metro pcs, even though its a t-mobile brand
be careful about buying cricket phones, legacy cricket iPhones are CDMA and they are not unlocking these, and the CDMA network is in the process of being shutdown. where as the post AT&T cricket iPhones are GSM and can be unlocked
wamart deal
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u need to go to walmart.com and buy the $35 starter kit
you get 100 minutes of talk + unlimited messages + 5 gb of data and music freedom for $30
thers a way to get this with a $6 amazon sim card, but you still have to pay $30 to activate it
this months stats
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tmoble data used = 4.24 gb
iPhone says
5.7 gigs used
music 1.2 gigs
wwe 2.8 gb
system services 1.1 gb
misc 552 mb ( this is Facebook, safari ,mail , speedtest, etc)
so there is definitely 1.2 gigs of music i am streaming that does not go against my cap