I buy wheat seeds in bulk, and then sprout them. This is less expensive than peas. Now, I still do buy peas, especially when they are on sale, but having wheat sprouts adds different nutrients and they don't need to be frozen.
I learned some weeds that are good for the ducks to eat, and I let them grow in places in the yard, so there is free salad. I also grow some of our own lettuce and other greens for treats.
I use the used duck bedding as mulch in the garden instead of buying mulch. And duck manure supercharges the compost, so I don't have to buy compost. I have already used about $100 worth of compost and good garden soil this spring from duck bedding and vegetable scraps that we produced here.
I don't buy chicken eggs while the ducks are laying - that saves money. Oh, and duck poo in water is a great, free fertilizer.