If you're going to go island hopping, it's waay better costwise to just buy a multi-destination ticket that includes your departure from the US as opposed to buying tickets straight out of DR. DR flights are expensive. For example, a flight from Richmond, VA to PR is about half that of one from DR to PR.
If you plan to go to the island of Dominica, make sure you make your multi-destination connect in PR on the way TO Dominica (for no reason besides to lower the price). You can do the connection for the same day as your arrival in Dominica if you don't actually want to be in Puerto Rico or not, if you want to stay in PR. You'll save about $300 this way. The ferry to PR is way too long and expensive, but cheaper than a round trip (as opposed to multi-destination) from DR. Expedia has been great as far as prices on multi-destination trips.
Below is more info about Dominica if you're interested.
Suba diving, whale watching, snorkelling, Cabrits Dive Centre, East Carib Dive, Anchorage, Dive Dominica, Nature Island Dive, Nature Island Destinations, Commonwealth of Dominica, West Indies - accommodation island-wide, vehicle hire, bird watching, and NationMaster - Dominican Crime statistics
Frequently Asked Questions about Dominica and it's Real Estate, land and property opportunities
The virgin islands are one of the more expensive locations as far as cost of the flight if you add it to a multi-destination set-up.
Hope this helped! I'm going to try to go there after I go to DR. P.S. the hotel most people stay at in Dominica is Fort Young Hotel. Check out their weekend specials and be sure to be careful about hurricane season. Good luck!