I did some simple calculations based on nutrition labels, a recipe I use, and google.
I use Anthony's Vital Wheat Gluten which is $18 for 4lbs (1.81kg) or $0.99 for 100g. At 77% protein by weight, that makes vital wheat gluten $0.013/g protein.
Chicken breast averages $3.18/lb in the U.S., or...