There are many SPA available in Bali. I'd learned that they come in 3 range, the hawker range, the mid range and the high end spa.
You can find a lot of hawker ranges spa on the road side of Kuta, legian, semiyak & ubud, this spa provide foot massage treatment to 1-2 hours of traditional body scrub spa treatment. price range from Rp50.000 for foot massage - Rp300.000 for body scrub treatment, but the private room area will not be that private...
The mid range Spa can be found in Nusa Dua, Tanjung Benoa and before sanur area, they are normally a building by it's self, the skill and treatment are of better quality as they train their therapists in a more professional way. They provide individual private rooms with bath tub for flower bath and shower. price range between USD50 - USD85
The high end spa, they are normally located in a 5* hotel. Like Ritz carlton, ritz spa, Thalasso in grand mirage, the Mandara Spa in Nikko, Ayung Spa in the hanging garden in ubud, the ocean front spa in bulgari resort, the price start from USD180
So it all depend on what you really want, but last time I tried the Red Wine spa in Halo Bali, on the way to sanur. I kinda like that one, the deco, the privacy and the treatment, they let you choose from green tea & chocolate body scrub and a bottle of red wine to enjoy while relaxing in the flower bath. price I paid, USD68 but my friend got us a very good discount