I don't get At&t's Mobile Share Plan?

Okay, so my dad has an iphone 4s and he currently pays about $85/mo for it. I'm planning on getting the iphone 5 16gb but my dad doesn't want the high monthly cost (for me).So i saw the "mobile share plan", i read it but i don't understand it. Can someone just explain to me how much we have to pay overall each month if we got At&t's "mobile share" 1GB plan?

$40 for 1GB of data
$45 for first smart phone
$45 for second smart phone

$130 subtotal

Then 25% of $130 for taxes and fees $33.

Total monthly bill $163 per month before any insurance or other add ons.

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