For creamy dressings, you've got a number of options for your base. If you compare calories of different bases, for one cup of ingredient you get
Mayo, Regular 960
Sour Cream, Regular 445
Greek Yoghurt, whole milk 260
Mayo, Light 240
Cottage Cheese, Regular 221
Sour Cream, Fat Free 189
Silken tofu 188
Cottage Cheese, Low Fat 163
Yoghurt, Whole Milk 140
Yoghurt, Skim Milk 137
To compare, a cup of Kraft Ranch dressing is 1184 calories.
So you can see there is a *huge* variation. For the above ingredients, some are thicker than others, so the actual calorie content of the dressing at the right consistency will be lower for thick things like cottage cheese or the silken tofu that need to be watered down a bit.
You can use dressing mixes, or make your own. For example, blend in grated parmesan, lemon juice, salt and fresh ground pepper, Wochestershire sauce and/or anchovy paste for a Caesar like dressing. Or puree yoghurt with cucumber, dill, onion, garlic and lemon juice for a tatziki dressing.
For really low calorie dressings, I'm fond of fresh squeeze lemon juice with salt and fresh ground pepper. Or mix lemon juice and zest with miso paste, or with anchovy paste, for an intensely flavoured dressing. And indian chaat masala powder is actually pretty good on salads - it's a blend of toasted ground spices and salt designed to be sprinkled on raw vegetables.