We just use a plain old $20 Osterizer and see no need for an expensive thing for smoothies only. It does everything.
The thing with smoothies is that there is no recipe! You just use up what you have and it's a great use for things that have gone a little soft (like the edges of cut green and red peppers that are getting a little too soggy for a salad but are not in any way moldy or past their prime). We tend to use regular products such as low sodium vegetable juice and then add what we want. Kale, spinach, peppers etc. are all good. We dilute with water too.
For dairy type bases, we use milk (any type) or thin it with water. We don't spend money on things like almond milk which are really overpriced - we just add sliced almonds to regular milk or soy milk. For fruits, we often add a little unsweetened pineapple juice instead of milk (it's a natural anti-inflammatory) and then add fruits as needed - frozen fruits work great and are much cheaper especially out of season. They chill the smoothie and chop up just fine. Using regular fruit, put in the tops of strawberries and parts of apple cores (maybe minus the seeds) and anything else you would normally discard. Again, use up fruits that are a little bruised or going soft. Don't stay away from fruit because of sugar - the fiber, antioxidants and other nutrients are really important. A too-soft avocado is a good addition.
For anything milk or water based, you can add plain unsweetened cocoa powder (virtually no calories), peanut butter, bananas or strawberries or other berries. (Anything that goes in an ice cream flavor is usually good with chocolate, including almonds and walnuts.)
For anything tomato based, I add in the tops of tomatoes, cucumbers, peppers, celery, onion, anything that would go in gazpacho. I add in wheat germ or ground flax seeds too. (And by the way, the blender is a perfect machine for gazpacho - a great way to use up the last tomatoes and other produce of summer! I add about 1/2 Tblsp of olive oil to the mix and it all holds together when chilled. I top it with some additional chopped veggies like onions and peppers.)
I don't use Benefiber as someone suggested - I don't think it's a good fiber product. I agree that extra fiber is needed even on top of what people get in their regular food, but I use something that has soluble and insoluble fiber together which is what food scientists recommend.
Don't forget to add what's at the bottom of a bag of nuts (the crumbs) or even what's at the bottom of a box of cereal (grape nuts is a good example) - it's all fiber and some healthy fats.
So go ahead and use your imagination - and don't bother with an expensive machine. Overall, a blender is much better than a juicer which deletes all the pulp and fiber from fruits.