Preparing chicken breasts are probably the fastest thing ever, and I really do not think anything is much cheaper. But other good sources are:
Cottage cheese, skim milk, Skyr, eggs and tuna (stolen from other answers).
From (Berin Loritsch) you could look into local sources of cheap and good protein.
Back to the breasts!
Buy a bunch of breasts.
Wrap them in bacon/thinly sliced ham.
Place them on a baking tray, with some baking paper under.
Cook them for 20 minutes a 180-200 degrees (around 380-400 fahrenheit?).
Cool off, and store in fridge.
It really does not get any cheaper or faster than that. You can slice them open and stuff things like cheese and spices in, to get some variation.