If your Air Transat flight is booked through a Canadian Tour Operator, the returm will be in 7 or 14 days even if it is air only. For a fee of $50 you can get it extended in the DR for up to 2 wks more. Return airfare from Canadian cities will vary from $C 250 to over $C 500 plus takes. The price will depend on which Tour Operator it is purchased from, seasonal demand, and how last minute you book. Prices start dropping 2 weeks before departure if there is a surplus.
If you stay more than 4 weeks, you will have to standby at the airport and buy a one way ticket. I paid US$ 225 for one of those in July. Years ago there was a thriving market in return tickets which you could purchase from people who knew they would stay more than 2 or 4 weeks. This has been stopped, because the charter airlines now compare the name on the ticket with the name on the travel documents more closely.
Air Transat does not book seats directly. Suggest that you get a good travel agent to shop around for you, then compare.