I think the bus would be cheaper.
Caribe Tours has a bus leaving for Sosua hourly from early morning to early evening. Take a taxi to Caribe Tours, then a bus to Sosua. Don't know the cost of the taxi, but the bus ticket will be RD$80, or about US$2.75. From Sosua to Cabarete, take a taxi. Not sure of the price, but figure RD$200-250. Figure US$8.75. Not sure what the taxi to Caribe tours costs, but I doubt if the total is going to be as much as airplane to POP, then still have a longer, more expensive taxi ride to Cabarete than you will if you take the bus to Sosua.
Figure 2 hours from the Caribe Tours bus terminal to Cabarete, including the taxi from Sosua.