I prefer protein powder over ready-to-drink shakes, and powder tends to be less expensive. My favorite brand of protein powder is Ideal Lean, but it is really a matter of personal taste. For ready-to-drink shakes, Equate brand is pretty much the same as Premier Protein but priced much lower. I am almost 2 years out now and I no longer drink protein shakes. I get plenty of protein from food, and I'd rather eat food than drink a protein shake.
It is really important to budget for what you'll need post-op, and don't forget about vitamins. The best value in bariatric vitamins is the BariatricPal brand multivitamin, which is $99.99/year for a subscription (I recommend you buy a smaller bottle first and make sure you like them before you subscribe; also, you'll probably need chewables for the first few months after surgery). Depending on your program/bloodwork, you may need additional vitamins (I take calcium, chelated iron, vitamin B-12, vitamin D, and biotin in addition to the multivitamin, and I also use pre-workout and essential amino acid supplements). Vitamins are super important for gastric bypass patients, so you have to make sure you're prepared to buy them before you have the surgery.
In the early post-op stages, I was eating so little that the costs balanced out. Now that I'm eating regular food and larger portions, though, I am spending a lot more on food than I was before surgery, even though I'm eating less. I used to shop sales and buy cheap food. Things like chips, crackers, cookies, candy, frozen meals and snacks, baking ingredients (sugar, flour, oil, butter), etc., can be very inexpensive when you buy them on sale. Now, I buy a lot of fresh produce, meat, fish, herbs, and spices, plus some healthier substitutions (sugar-free sweeteners, protein bars, low-calorie salad dressing, sugar-free condiments, olive/avocado oil, coconut flour, etc.) and prioritize health and taste over price. I'm sure I could spend less and still eat healthy if I really needed to, but I have invested so much into this surgery and losing weight that I'm willing to spend more money on food that helps me stick to my plan.