The Gold's Gym is easily the nicest one in the POP/Sosua/Cabarete area, but also the most expensive. It's about 300 pesos for the day, and a few dollars more if you want to use the pool. However, you get to use the entire facility (gym/pool/restaurant/beach/etc..., which is actually very well put together. The equipment is very well taken care of, and the cardio machines on the second level are set so you can fully appreciate the absolutely breathtaking ocean view. Lockers, showers, etc. are all what you'd expect at a better gym.
As a day pass in an okey-doke gym here in Atlanta is around 15 bux, I'm happy to pay the price.