Companies like Glanbia and Hilmar start with high-quality milk. And what they charge for the protein powders that come from that high-quality milk is based on what the milk costs. In today's market, that means that the prices for raw milk-based protein powders are high. Protein manufacturers will pass that cost on to consumers, meaning you'll pay more for protein powders that use high-quality raw protein from reputable protein manufacturers.
Unfortunately, not every supplement manufacturer goes to the reputable protein manufacturers. To cut costs and make their protein powders more affordable, some supplement manufacturers use questionable protein suppliers. Sure, they pay much less for the raw protein, which saves you a ton of cash when you buy their jug of protein powder, but the problem is that that protein powder likely contains far less protein and far more carbs and fat than claimed on the label. More frighteningly, these lesser-quality protein powders may also contain impurities and contaminants.
Remember the melamine-contaminated milk scandal in China? Because melamine is high in nitrogen content, as is protein, it can be added to milk that has been diluted to bump up the nitrogen ("protein") content of the milk.
The easiest way to spot a protein powder that is using cheap raw protein is its price. If it's much cheaper than the major brand protein powders, you better suspect that something is up. If you think that you're getting a great deal on a cheap protein powder, that's all that you're getting. You are NOT getting a good-quality protein powder. There is only one way for a company to undersell all other reputable companies, and that is by buying inferior protein. So be careful of protein powders that are not major brands, that you can't find on bodybuilding.com, GNC or The Vitamin Shoppe, and are incredibly cheap. As they say, you get what you pay for.
In addition to price, you can also distinguish a low-quality protein powder by its percent of protein. While most quality whey protein concentrates are somewhere around 70-80%, there are also whey protein concentrates that are as low as 35% protein. That means the majority of the protein powder is carbs (lactose) and fat. If a protein powder lists whey protein concentrate first on the ingredients list, followed by whey protein isolate and/or whey protein hydrolysate, but has less than 70% protein per serving, then it only has a very small amount of whey protein isolate and/or hydrolysate in it. Some companies add minuscule amounts of whey protein isolate and hydrolysate just to claim their products contain them, but the amounts are so small that they're not going to do much for your body.