The U.S. Department of Agriculture has added another item to liver's long list of attributes. It's cheap.

Liver's other virtues are well-known. One serving can supply a woman with half of her daily iron requirement (iron is a prevalent deficiency among pre-menopausal women). One serving supplies at least a week's worth of vitamin A. The B vitamins are there in quantity, minerals are profuse and calories compare favorably to those in an equal amount of turkey or chicken. Liver cooks quickly, and there's virtually no waste.

In a December survey of Washington-area meat prices, the USDA found beef liver cheapest among all meats surveyed (it averaged 93 cents a pound). Liver compares favorably even among other meat alternates. While the protein of dried beans costs half as much as the protein from liver (11 cents versus 22 cents for 20 grams), you'd have to eat more than 8 ounces of dried beans to get the protein available in about 3 ounces of liver.

Turkey and whole chicken are, besides beef liver, the most inexpensive meats, followed by both regular and lean ground beef. Frozen perch fillet, while costing $1.85 a pound, yields more edible meat to the pound and therefore ranked fifth. Not surprisingly, the five most expensive meats were foods that are probably just a dim memory--center-cut pork chops, sirloin steak, beef rib roast, porterhouse steak and loin lamb chops.

Even the most ardent calves' liver fans, though, sometimes balk at beef liver, which tends to taste stronger and cook a little tougher. The following recipe uses shallots and wine vinegar to balance the pungency of the liver and has won new supporters for beef liver. Not only can the meal be cooked quickly, it can be purchased quickly. You can get through the express lane of the supermarket with the entire menu provided you have flour, sugar, salt, pepper and butter and/or oil at home.

EXPRESS LANE LIST: beef liver, shallots, red wine vinegar, rice, parmesan or cheddar cheese, parsley, yellow squash, oregano. QUICK BEEF LIVER (3 to 4 servings) 1 pound fresh beef liver Salt and pepper to taste Flour Butter 3 to 4 tablespoons chopped shallots 3 to 4 tablespoons red wine vinegar

Dip beef liver into flour that has been seasoned well with freshly ground pepper and a little salt. Heat enough butter in a skillet to coat the bottom of the pan. Cook beef liver quickly over high heat--about 2 minutes per side (less if you like it rare)--and remove from skillet. Set aside. If the butter has burned, wipe the skillet with a paper towel. Add fresh butter to coat the bottom of the pan and add chopped shallots. Cook over medium heat until they begin to brown. Add vinegar and swirl over heat to reduce to 1 or 2 tablespoons. Add a tablespoon of butter to hot vinegar mixture and pour over liver. Serve at once with rice and steamed yellow squash that has been seasoned with oregano. RICE ROGERS (3 to 4 servings) 1 cup uncooked white rice 2 tablespoons butter 1/3 cup grated parmesan cheese (substitute cheddar, if desired) 1/3 cup finely chopped parsley

Cook rice according to package directions. Add butter, cheese and parsley to hot rice. Stir to combine well.