WASHINGTON HAS lost its dubious distinction as one of the most expensive cities in the United States in which to buy food. But even here, anyone willing to shop around has always been able to pay a lot less for the same products. Especially during the summer. And especially if they are willing to spend a large proportion of their food budget on fruits and vegetables.

For a number of years, Magruder's has offered produce at prices that are so much lower than the supermarkets' it seems they must be selling them at a loss. In the last six years, the number of produce vendors parked along major arteries into the city has increased dramatically. Their prices almost always are a little less than the supermarkets.

And now that the Farmers Market at RFK Stadium has joined the smaller Farmers Market in Silver Spring, vegetables and fruits are truly a bargain. f

There is no way to document the effect such markets have on prices at the supermarkets. Perhaps they have none. But a price survey taken on July 10, shows that the places to buy fresh fruits and vegetables are RFK Stadium and Magruder's. You can save a good deal of money.

At RFK, 18 different vegetables and fruits cost $7.21. Equivalent amounts of usable vegetables at Magruder's cost $7.37; at Safeway, $12.05; at Giant, $10.63.

That's price alone, without taking into consideration freshness (and freshness is related to nutritional value) and quality. The tomatoes at the Farmers Market and Magruder's are local; they do not come from Texas or California or Florida, and they are not gassed to make them turn red. All of the fruits and vegetables at the Farmers Market are from within approximately 150 miles of Washington. That's one of the prerequisites for selling at the market, which is open Tuesdays and Thursdays from 2 to 8 p.m. Another is that only 10 percent of the items a farmer sells can be from somewhere other than his or her own farm. There are several hucksters there who are not selling things grown on their own farms (there is some question about whether or not some of them are even farmers), and officials say they have warned them. Next, they will throw them out.

There are some variations in price and quality among the different trucks, so even there it pays to shop. Also, there are weight variations between the Farmers Market and others. For example, the small cantaloupes at the Farmers Market weighed at least a pound more than the melons at the other markets. Only the edible flowerettes and tender stems of broccoli are sold at the Farmers Market; the thick, tough stems have been removed. A bunch of beets at the Farmers Market is twice the size of a bunch of beets at the supermarkets.

For those who want bargains and fresh air and the feel of an old-fashioned market, Parking Lot 6 at RFK stadium is the place to be. For those who want bargains and the crowds of Filene's Basement, Magruder's is the choice. For those who want convenience above all else, the supermarkets will do.

If, after you have purchased all these wonderful bargains you want to know what to do with them, perhaps some of these recipes will give you inspiration. CUCUMBERS AND MUSHROOMS IN YOGURT (6 servings) 3 cucumbers 1/4 pound fresh mushrooms 1 cup plain yogurt 1 large clove garlic, crushed 1 tablespoon olive oil 2 tablespoons chopped fresh mint 2 tablespoons lemon juice Salt and freshly ground black pepper

Peel and slice cucumbers and place in a bowl. Slice mushrooms thinly and add.Mix remaining ingredients to blend. Combine with vegetables. Season to taste with salt and pepper. Chill until ready to serve. RASPBERRY GRANITA (4 to 6 servings) 2 cups fresh raspberries 3/4 cup dry white wine 1/4 cup creme de cassis 1 tablespoon lemon juice

Combine all ingredients in blender or food processor. Blend until smooth. Pour into sectioned ice cube trays and freeze at least six hours, or as long as 12. For each serving place 4 or 5 cubes in blender or processor. Blend or process 30 seconds. Spoon crystals into glass. BAKED POTATO, VEGETABLES AND CHEESE (3 servings) 6 small baking potatoes or 3 large 1 medium head cauliflower 6 carrots 3 cups coarsely grated cheese, such as sharp Cheddar or milk, Meunster, Monterey Jack 1 1/2 cups plain yogurt Salt and freshly ground black pepper

To save time, smaller potatoes may be used. Place metal skewer through them to speed up baking process. Smaller potatoes will be baked in 45 minutes at 425 degrees; large potatoes will take one hour.

While potatoes are baking, break cauliflower into small flowerettes. Scrape carrots and slice on the diagonal. Fifteen minutes before potatoes are ready, steam the carrots and cauliflower.

To serve, cut open the potatoes and place in deep individual serving dishes. Top each serving with 1/2 cup yogurt. Spoon on the vegetables and top with 1 cup grated cheese for each portion. Serve with salt and pepper. HOT POTATO AND BROCCOLI VINAIGRETTE (4 servings) 3 medium potatoes, about 1 pound 1 pound broccoli, trimmed and broken into flowerettes 1/3 cup olive oil 5 tablespoons cider vinegar 1 clove garlic, finely minced or pressed 1/2 teaspoon dry mustard 1/4 teaspoon paprika Salt and freshly ground black pepper to taste 2 green onions, finely sliced, white and green part

Cook potatoes, whole in their jackets, until just tender. Drain, peel and cube; keep warm. Meanwhile steam broccoli until just tender; cut into small pieces; keep hot. While vegetables are cooking combine remaining ingredients in small pot and bring just to boiling; stir. Arrange vegetables in serving dish; pour over vinaigrette; stir gently and serve. PEACHES STUFFED WITH TOASTED ALMOND CHEESE (6 peach halves) 3 medium peaches, ripe and juicy 3 ounces cream cheese, softened 1/3 cup (2 1/4 ounces) slivered almonds, toasted

Cut each peach half and remove pit. Enlarge the cavity by removing a little of the peach pulp. Reserve pulp for other use or EAT!

Combine the cream cheese and almonds and place in the cavity of each peach. Chill until serving time, but do not serve chilled.

For best flavor, remove peaches from refrigerator 3/4 to 1 hour before serving. Decorate with green leaf.