Cellular plans for iPhone4s

my grandmother open line me on cell phone plan at&t , plan on getting iphone4s.
question how phone going cost when buy it? keeps saying $600 know that's not right because we're opening new line.
also, know how start? know line $10 , isn't $30 because it's smartphone?
, how think bill every month? i'm sure we'll share minutes , have unlimited text , 2gb of data.
accurate information appreciated.

thank you

 

we need more information answer questions: (country)? carrier? etc.

 

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