I believe the last "Bavaro Express" bus for SD leaves at 4:00. That's gunna be a tight connection to say the least, considering you would need to claim your bags, clear customs, and get a cab to the bus station in Bavaro.
More convenient, but obviously more expensive, would be the 6:30 flight from Punta Cana to Herrera airport in SD. $80 one way
http://takeoffweb.com/_wsn/page2.html
As far as I know your only other option would be to spend a night in Punta Cana or Bavaro and catch a bus the next morning.