Let's look at this a little bit more
If you get in on time you can make the 8 pm bus to Santiago, stay at the Hotel Colonial for 15 dollars and take the first bus to POP at 6 in the morning.
Total cost should be around $30 including the taxi ride from the airport to Caribe tours (Sorry, I have no idea how much those rides are.)
But lets say that the taxi is RD$800 to Santo Domingo, the bus is what nowadays? RD$150 pesos to Santtiago? and another 80 to POP? Taxi in town is RD$70 pesos.
And you have nearly $100 dollars left over for beer!
Think about this alternative. OR
Stay in a little hotel in Santo Domingo for the night and take a bus the next morning. No matter you save a lot of money..
HB, who would walk before spending that much money to travel across the country at night for nothing....