We'd have to determine what is an "additive". Many of these would be difficult to consume without flavoring and sweetener. Many have recognizable minerals and functional lecithin and fiber added. I included the Designer Whey, it's full of functional additives (vitamins etc).

We looked into whey protein isolate a while back, that presumably has very few additives (in some cases, only a tiny amount of lecithin):

Without looking too hard, 5lbs of chocolate whey protein powder is is $56 on Amazon... (these aren't isolates, just decent)

https://www.amazon.com/Dymatize-Elite-100-Protein-Chocolate/dp/B00CUDYY2U/

https://www.amazon.com/Pure-Protein-100-Whey-Powder/dp/B002XULC5W/

https://www.amazon.com/Sports-Nutrition-Protein-Isolate-Unflavored/dp/B0015AQL1Q/

https://www.amazon.com/Designer-Wellness-Probiotics-B-Vitamins-Gluten-free/dp/B001CCUIX4/

...

(isolates:)

Dymatize Elite - https://www.amazon.com/Dymatize-Elite-100-Protein-Chocolate/dp/B00CUDYY2U/

Notice the boosted Potassium content on the chocolate flavour, a plus.

https://dymatize.com/elite-100-whey

...

Here is another option: https://www.amazon.com/Protein-Isolate-BulkSupplements-kilograms-Unflavored/dp/B00E7IODXQ/ however, the NOW is a bit more economical.

...

Finally, I'll point out that eggs are back on the menu and nearly as good as whey. Liquid egg whites in the carton still work out to around 3 cents/gram protein (similar to protein powder). They may drop in price soon. Whole eggs are even more economical, around 1 cents per gram protein. And of course, no additives here.