I did something similar during the first promo several weeks ago: my wife was eligible for an upgrade to her 8GB iPhone 4. My daughter, who had a 16GB iPhone 4S, was able to use my wife's upgrade to get an iPhone 5 (in the process, giving my wife her iPhone 4S). Everyone got an upgrade from their present phone.
Right now, I think BestBuy's (and Apple's) imperative is to clear stock. Don't know if they are doing this solely to prepare shelf room for the next iteration (whatever its called) or whether they are re-using parts or materials. Whatever the reason, everyone seems pretty aggressive about buying these units back. For that reason, I don't think you'll have any problem getting the full $150, if not more. The question for you, though, is what you will have to come out of pocket for for the iPhone 5 16GB. You get a $50 instant rebate + the $150 trade in gift card for a total of $200. Since the iPhone 5 16 GB model retails for $199, you should be good to go (you'll have to pay the sales tax and any carrier activation charges; in my case earlier this month, that was about $13 total).
Good luck! Be prepared to spend some time in line if you go this weekend, though.