By "off network" I think he wants to go to somewhere that he can then buy a seperate ticket to Cuba from. I seem to remember that Cancun is about $250 on Mexicana, Grand Cayman is about $175 on Cayman Airways, and Aruba is about $200 on Aeropostal. All the others run about $500-600.
I do hope youre going to put more than a week into the LSAT. A lot of it is natural logical ability but a lot is also question pattern recognition that comes only from doing a slew of them before the test. That's what Kaplan and the other test prep places actually teach and it's what seperates the 170's from the 155's. Whatever you're getting in your practice tests, drop the score 5-10 points and that's about where youre going to end up at. In any event, good luck with it.