First let me set the record straight!
There are only FOUR places/services were the price is SET in the DR!
1. The Tourist Card (US$10)
2. Bus fares on Caribe Tours/Metro. (various)
3. National Supermarkets (you can't negotiate in Playero or Supermercado National)
4. Phone Cards for cell phones.
Everything else from Hotels, Food, Chicas, Cars, Taxis, etc. is negotiable! Those prices will depend on repeat business, Spanish ability, which way the wind is blowing, etc.