Are you flying into Miami for $200.00? Seems like GR would be in the same price range? I have been looking for tickets for a month for late Feb early March.
Since you didn't say anything about money, I would go to Belize, I have never been, but I hear great things about Ambergris. PM "El Pescador", he knows a thing or two about that area.
Your not a US citizen, correct?
Or sneak into Robbies at night with a french fry fly.
I don't ever go near the Miami Airport. I had some bad situations develop there years ago and the Miami area drivers are the worst in the US. I fly Air Tran non stop from Milwaukee to Fort Lauderdale for $194.00 round trip. Sometimes, the price is even lower than that. GR might be similar, but I'm not sure you'll find a non-stop flight. Fort Lauderdale is only about 45 minutes farther north than Miami and you'll save a bunch of that time by taking the Turnpike and avoiding all the Miami traffic. It's what I've been doing since the late 1990s. It used to be on Midwest Airlines, but they don't fly to Fort Lauderdale any longer, or any place in South Florida for that matter.
You really won't save any money by going to Belize. You'll be fishing from pangas instead of a flats' boat, and their flats are very soft. Even a little bit of wind will cause them to color up and there goes your tarpon fishing. They then can take you to some mangroves to fish for small tarpon and snook, but usually they'll take you to a hard flat to fish for small bones.
I'm not that familiar with tarpon in Cuba, but I've heard great things about the bonefish and permit in CERTAIN areas. You really have to do some homework on that one as to where and when to go. In reality, there's nothing like southern Florida for tarpon. From Jupiter on the East Coast, through the Keys, up all the way to the panhandle on the West Coast can be superb if you pick the right time of the year for each area and do your homework before hiring a guide.
Upon proof reading this I realize I sound like I work for the Chamber of Commerce for southern Florida. Just to clarify things, I don't!