I am currently an AmeriCorps member, which means I eat/live/pay for medical stuff on $12,000/year. So basically, I have SO MANY thrifty ideas for you
1.
make your own probiotic yogurt (if you eat it): this is a great protein addition in smoothies, it is VERY cheap (1 gallon of yogurt = price of one gallon of milk), it is VERY easy, and most people feel that probiotic foods work better than probiotic pills because the food works as a vehicle for the bacteria to live. You can use the yogurt in your smoothies directly or you can also stain some of your yogurt into a probiotic labneh (sort of the texture of cream cheese) and use the whey that you have strained in your smoothies. There are TONS of online tutorials, but basically just heat milk to about 185 degrees, let it cool to about 110, stir in a couple of tablespoons of plain probiotic yogurt (can be store bought) and store for about 12 hours in a warm place. Thats it!
2. I would suggest figuring out what fruits work for you and then either buying a bunch when they are in season and freezing them yourself, or buying them frozen. (Store-brand on frozen fruit isn't that expensive) I bought a whole case of peaches at wholesale price last summer and pureed them and froze them. I am still using the puree (just peaches and a little vitamin c powder, no sugar or additives) and it tastes fresh and delicious. I freeze fruit or fruit purees flat in plastic freezer bags so that it is thin enough that I can break off a chunk.
3. Sometimes frozen cans of 100% juice concentrate go on sale at the grocery store for $1. Make sure you get 100% juice ones so you aren't putting added sweeteners in your smoothies, but you can scoop some concentrate right into the smoothie to add healthy flavor.
4. Bananas are SUPER cheap and have lots of calories so they are a great way to make a smoothie go farther. You can also freeze them when they get brown, it gives them a great texture.
5. Oatmeal- you can put dry oatmeal in the blender and powder it up to put into your smoothie. It absorbs pretty quickly that way and makes your smoothie thicker and more filling. It is also VERY cheap.
6. I always have spinach on-hand because it is absolutely great for you and even in a flare I can handle a few leaves in my smoothie. It is pretty cheap fresh or you can buy frozen and just break off small chunks for your smoothie
7. Other items that can be added that pack a punch but aren't too expensive if you shop around: natural peanut butter, flax seed (can be put in freezer), apples/applesauce, any greens (very easy to freeze or you can buy frozen), oranges
The things I have listed above are more baseline smoothie items. Some things that are great in smoothies are expensive. Avocados, coconut, chia, acai berry, wheat grass etc. are all things you can add just a little of when you have them. You are still getting great value from simple oatmeal, bananas, peanutbutter, yogurt, etc. Another way to make smoothies more delicious is adding things like cinnamon or nutmeg.
Sorry this is a little rambly, but I hope you found at least a few helpful suggestions! Good luck!