Please let me know if I am correct about this.
Currently I have a family plan with 2 smartphones (GNex and iPhone 4S) and unlimited data on both lines. My plan is to add 2 lines when I am ready to get new phones for those lines so that I can get the subsidized phones. Here is how I worked out the math, but please let me know if I am wrong in my thinking.
For my GNex, I will replace with another high end Android GNex equivalent, which have been running $650 full retail at launch but $300 subsidized. Adding a line online I think there is no activation fee, but you pay taxes on the subsidized phone price and the line which brings the phone where I live to $318 and the 20 months of the new line to $300 (since I would activate a basic phone on the new line and drop the data). So a subsidized high end Android GNex equivalent would cost me $618 if I had to add a line to get it. To buy full retail including the tax would be $689.
Thus, the savings would be only $71 + the cost of a basic phone (which I already have so is free), but of course I would have the use of an extra line with a basic phone on it if I ever needed it (for instance if my phone broke and I had to wait for a replacement).
Now for the iPhone it seems more worthwhile since the 16GB storage models have been retailing for $650 but the subsidized price is $200. To buy full retail with tax would be $689. To add a line and get a subsidized iPhone with 16GB storage would be $212 + $300 = $512, a savings of $177.
To Summarize (I realize the prices may go up in the future but using current data):
Full Retail Android GNex Equivalent = $689
Subsidized Android GNex Equivalent = $318
Additional Line for 20 months = $300
Savings $689 - $318 - $300 = $71
Full Retail iPhone 5 w/16GB = $689
Subsidized iPhone 5 w/16GB = $212
Additional Line for 20 months = $300
Savings $689 - $212 - $300 = $177