Very good question. Unfortunately, the calling card market is a known to be a racket. What all the companies do is sell the cards and advertise cheap rates and then and some point change the rates wherby people quit buying them and they are discontinued so the company creates more, etc.
Another trick they are doing now is saying there are no connection fees but they charge a hang up fee instead.
If you want the best rates unfortunatley you will have to do some experimenting. First, try to go to bodegas owned by Dominican's and ask them what are the best. Then buy the $2 cards first and give them a try. FYI, typically a good $5 card will give you 150 to 200 min. and maybe more than an hour to cell phones.