They all vary. To go on the new highway out of Samana was 500 pesos. We went from PC to samana and back in early January and the total cost was approx 1900 pesos (42 USD). It was well worth it when you compare it to the old highways
If you are going from Santiago to Samana City, there are no tolls. If you head to Las Terrenas instead of Samana City, there is a toll road -- recall it is RD$450 each way.
From Santiago to Santo Domingo there are no tolls. The toll is RD$30 but it is in the south-north direction.
I believe it is during the peak of Easter Week that the government gives up collecting tolls in order to make traffic flow. I believe that is the only time it gives up collecting road tolls.