If I had to make a guess. You could probably get a Finals ticket for the earlier games (like 1-3) maybe starting around $300. Of course it depends on what teams are in the finals.
I would say around $300 would probably get you one ticket in the very last row.
I lived in Dallas in 2006, and tried to get tickets for the Finals for the first game. That's about what last row tickets were going for.
I was actually at Game 6 in Boston in 2008, very last row, and the face value of my ticket was $180 I believe. Looked at how much they were going for (because if they were going for a lot I would have considered selling it) and I think they were only going for like $300ish then too, but I may be remembering wrong. I'm sure others on here tried to go to Finals games in 2008 and 2010 and would know.
Tickets might be even tougher to get in a city like Miami though (or LA even, but I don't think they're a threat). A lot more celebrities in those spots driving the prices up. Plus a lot of people like yourself, willing to travel to Miami with hopes of getting tickets, but if not you'd be happy just sitting on the beach. Not as many people would travel to places like Dallas or Boston to try to get a ticket but would happily settle for the beach or a vacation if things didn't work out.
If you want you want the most affordable Finals ticket (and by affordable I mean $300ish), plan your vacation to OKC