We have 3 airlines serving Santo Domingo from South Florida, so competition galore. Even a 3 hour drive for me to FLL airport seems more and more worth if if I get the $1.50 or $9 fares I get on Spirit. AA and JetBlue keep fares reasonable on these routes as well in order to stay remotely competitive with Spirit (figuring in add ons for baggage)
You are correct about there being more flights from NYC. As far as cost of flights, that has been the case historically, but not so much recently. I just compared the cost for round trips from MIA-SDQ and JFK-SDQ on 3 seperate sets of dates in May, and it was an average of about $50 cheaper from Miami. It looks like the cost of fuel has now risen enough to overtake the New York advantage in passenger volume. COL is indeed lower in South Florida than in NYC, and I would say more than a little.