If I understand correctly and you are trying to get to either Montreal or Quebec City from the DR, then go to one of the charter companies that have already been listed. They will sell you a one-way ticket back, if there are any seats available and there usually are in the summertime.
The last time I did this (3 months ago) it cost me $500 from PoP to Toronto and I believe it is the same fare for Air Transat to Montreal. Don't know about Quebec City.
I have always checked out all flights whenever I try to go back as I used to buy Air Only from Canada to the DR and then just not go back, therefore forfeiting my return ticket. Many of us do this and there are always seats available. The worse that has happened to me was once I had to wait one more week.
The only problem with this method is that you will not know for sure until the week you want to go. However, you can check all charter airlines and see which one comes up with a seat. So far, (last time I went) they all charge the same. It was $400 last year, but they raised it.
I have never ever known Air Canada to have reasonable fares either way and unless I can use my points, don't even bother.