Why a 30 gram dose is the comparison unit
Comparing foods by cost per 30 grams of protein normalizes for the fact that protein density varies enormously: a serving of lentils and a serving of whey deliver very different amounts of protein per gram of food. Pricing each source at a common protein dose lets eggs, milk, lentils, chicken, beef, salmon, and whey line up on the same axis.
Thirty grams is a convenient unit because it sits near a typical per-meal protein target for many adults.
What drives the spread between cheap and costly sources
In most markets, plant staples like lentils and dairy such as milk tend to be among the cheapest per 30 grams, eggs and whey are mid-range and protein-dense, and animal flesh, especially salmon, sits at the higher end. Exact figures shift with region, season, and brand, so any chart like this is a snapshot rather than a fixed ranking.
Cost per gram of protein is one input among many; protein quality, satiety, micronutrients, and overall diet also matter. This is general nutrition and budgeting information, not dietary advice tailored to any individual.