In the US, the hold on my credit card has always been for the amount of the estimated charges and no more.

In the Dominican Republic, Avis' site seems to say that if your credit card doesn't have car rental insurance, they'll put an extra $1,000 hold on your card, but then goes on to say that they don't do that if your card provides insurance.

It'll be interesting to see what documentation they require when I'm there in June, as my Capital One card has a stupidly low limit (only about twice the expected rental charges), and I'd prefer to use it if possible since they won't charge me foreign transaction fees or inflated currency conversion charges like Amex does.