Of the higher protein feeds, field peas are probably the least expensive & easiest to deal with. In all fairness, I must add that soybeans are higher and may be cheaper; I, personally, choose not to use them for several reasons. Flax is good - but not in a fermented feed because the hulls are flat & very smooth - it sticks to *everything* ; I feed it dry free choice, and it isn't calculated into my mix because they aren't that crazy about it.
This link to a simple calculator has a sidebar listing the average protein percentages of various foods:
http://homesteadapps.com/app/free/feedcalc/pearsonsquare.php
my suggestion would be to go through the list and see what's available and affordable in your area, run some numbers and come up with a blend that works. While it's a PITA, remember that using a larger variety of feedstuffs in the mix also results in a larger variety of micronutrients; this calculator is not useful for multi-ingredient mixes - it's linked because it has the easiest-to-locate and most extensive list of feeds I've come across online.
I also 'top dress' my FF with Fertrell Nutri-balancer to make sure I have all the bases covered; NOT mixed into ferment stage because 1. The mineral granules all sink to the bottom and are not evenly distributed, 2. It results in massive mineral build-up on surfaces of the container. Sooo.... it's in a huge 'salt shaker' beside the feed station