Corn is the cheapest feed you can buy. Though I wouldn't recommend feeding a wild flock. There are many reasons why That I won't go into at this point. If you want to help your local wildlife i recommend planting winter wheat in the fall. It grows like crazy, winters well and then produces during the spring bringing a plethora of birds and turkeys love it. It is high in protein at a time when turkeys need it the most. Wild turkeys need to free range, their diet is diverse corn is great as a winter feed because it is high energy but in summer contains to much fat. Sunflower seeds are 3 times as expensive and wheat is available mainly in late summer and winter. Oats is okay as well. But having a flock become dependent on a humans intervention is a risky game. Enjoy them when they come around but you shouldn't encourage them with food.