I believe you can keep unlimited data by purchasing a new phone from Verizon at full off-contract price as well, as unpleasant as that sounds.
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InfinityDevil
You can buy a used phone from cl, ebay or whatever. You can also buy a CPO phone at a price discounted over full retail and carries a full 1 year warranty.
If you have a legacy family plan, you should consider adding a line, seeing how the cost is. For example, I got the Note II recently for $150 at BB pm to Amazon. 20 months of service is $10, but after tax, closer to $15 for a dumb phone, so $300 for montthly service. $300+$150 = $450. Note that the contract is for 24 months, but you can upgrade after 20 months. $450, while a lot, is still cheaper than $700 for a full retail Note II.
You would need to be sure to either put a dumb phone on the line, or ask Verizon to put a phantom line on the line that you don't plan to use.
Tax depends on your area, so yours may be higher or lower than mine.
Sent from my Verizon Galaxy Note II