I have purchased 2 different TV sets both at Plaza Lama in Santiago. They have the same prices as Santo Domingo but much better pricing then the puerto plata area.
I have found that TV's are still really expensive in The DR (not sure why). It seems like any little old TV will cost you upwards of 500 USD's. If it's flat panel, forget about it.
Last time I went I stopped by Circuit City and bought one of those really slim 15"" for about 280.00 USD - I'm sure you can get them cheaper if you look around. I put it in my carry on suitcase and took it with me down there. My sister in Philly sent a 27"" door to door for 100 USD.
So my advise would be that if you can buy in The US and take it down there it is a much better proposition.
If you can to buy your tv in usa and use a shipment company to bring it here will be cheaper. To buy a tv here will cost you twice including the shipment.
In Puerto Plata the only resonable places to get TVs are Gonzalez Muebles, Casa Nelson if new ones. But it is expensive here.
Also, if you don´t need a new, a compra venta ( pawn shop ) like Compra Venta Antonio close to CaribeTour is an option. Much cheaper and not very old stuff.
A week ago I bought a Playstation 2 for my son there, for 3000 pesos. New price is 8000 here.
As good as new.