Natalie - My husband is from Higuey so we go there all the time. I believe there are excursions that go into Higuey I just don't know any of the details. Taking a cab might be expensive, just be sure to work out the pricing details before you get in. I believe the cab will even wait for you while you shop.
All of the prices are in Peso's. If you exchange money into Peso's you get a better exchange rate than you would paying US$ in the store itself.
If you wondering about how safe it is, it's definitely safe, and busy. Be sure to stop by the Basilica to get pics as well. I don't know if you can go inside anymore. I thought I read somewhere that they closed it to tourists because it interfered with the locals who go to pray and pay their respects to loved ones past.