Doing The Lobster Tail Math

I've been excited about the Craig's lobster tails from Maine that Whole Foods has on sale for $6 each (technically two for $12, but you can buy in any quantity).  The fish guy told me these are four ounce tails, a decent size.  At the Pearl Street store, I also found eight ounce packages holding three Craig's lobster tails on sale for $17.99 a package.  (Hope I'm remembering these details accurately, as I didn't write anything down.)  So do the math before buying.  Two four ounce tails equals eight ounces, and you can get them individually at $12 total for the two, meaning you'd spend 50% more if you pick up the package to get your eight ounces.  I used to think this stuff was obvious, until I encountered a woman who was about to buy a one pound package of fresh mozzarella at full price, when I pointed out that the half pound packages were BOGO (so she could get a pound at the price of the half pound).  She considered this for a bit, then asked me "Is that a better deal?"  I'm not kidding.