COFFEE PRICES SLASHED! read the banners across the supermarket windows - and coffee prices are indeed slashed. Sometimes by as much as 10 cents a pound. That means coffee now costs 10 cents less than three times what it did in 1975.

So don't drink coffee. Drink tea instead.

And if you think tea is lukewarm insipid stuff that comes in little paper teabags - and taxes like paper tea bags - buy your tea in Chinatown, where they take tea seriously.

When the Chiang Kai-shek government fled to Taiwan, it was careful to take cuttings from all the tea plants in China. But that was about as successful as Nixon would have been if he fled to Canada with cuttings from all the orange trees in California. Oranges don't grow in Canada and tea does not grow well in Taiwan.But now mainland tea is back - all the stores in Chinatown carry at least five or six different kinds. It tastes good and it is extraordinarily cheap. A budget of $5 or $6 an afternoon in Chinatown, and you will find yourself with more kinds of tea than you can drink up in a couple of months.

I found that prices varied widely for the same teas - the cheapest seemed to be at Tuck Cheong, 617 H St. Nw (628-2224) and Wang's, 800 Seventh St. NW (347-2447), noth run by the same family; and at Da Hua Foods, 615 I St. NW (638-2992), the largest and newest store in Chinatown.

The Chinese have communized tea production, but have not abolished brand names.In fact, besides brands, there are a number of varieties of tea (like pu erb, oo long ), and a number of sub-varieties of each variety (tikuanyin oo long, minb pei oo long ). It's a little confusing at first, but prices are so cheap you can afford to experiment.

In general, the best brand is Sea Dyke, and it is also the most expensive - sometimes as much as 2 cents a quarter-pound more. Some teas come in brightly lacquered tin containers, and these cost most of all - a little more than double the price of the same tea in cardboard. Tea in tins doesn't taste any different, but it makes nicer gifts.

Here are the kinds I found, along with prices and a little description (or warning) about what you can expect them to taste like:

GON JIM (40 cents a 2-oz. pkg.), Gon Jim is the cheapest tea in Chinatown and not highly prized by serious tea-drinkers. You will recognize it is what you ordinarily get in Chinese restaurants - called oo long , and sometimes mixed with a little real oo long .Worth trying because it is the Chinese equivalent of government cafeteria coffee.

JASMINE, Sea Dyke brand (65 cents a quarter-pound in paper, $1.84 in metal tin). I've always hated jasmine tea because it tasted like perfume mixed with wood chips, but this is real jasmine tea, made with a fine black Chinese tea and lots of little petals, and it actually tastes good enough to drink.

ROSE CONGOU, Sunflower brand (85 cents a quarter-pound, paper only). This is the same sort of stuff, flavored with rose petals instead.

OO LONG. I got four different kind of oo long - all of them tasted somewhat similar, but there was at least as much difference as there is between Colombian and Brazilian coffee. minh pei oo long , Sea Dyke brand ($1.25 a half-pound) was the mildest, with a sharp, very redolent taste, and the best to me ("much preffered by vast overseas Chinese" said the label). Tikuanyin oo long , Sea Dyke brand (90 cents a quarter-pound) was much stronger and more tannic, with a flat heavy taste. Shui bsien en oo long is a mediumheavy type, somewhere between the first two in strength and tannin; I got two brands. Sunflower (89 cents a quarter-pound in paper) was mild and tasty; Dunhuang ($1.99 in a big, beautiful tin with floating mythological creatures of some kind on the side) seems to be the cheapest - maybe that's why the can decoration seems to blatantly counterrevolutionary - and to me the worst, a very stewy and even tarry drink.

Chinese oo longs do not taste like American pekoes, or like the expensive European blends you buy in gourmet shops. They are - mostly - milder, with a clearer and brighter taste. If you are really adventuresome, however, you might want to try more exotic stuff. Here are a few:

PU ERH ($3.50 a pound). Pu erb is a compressed tea, made into a big flat wheel, the way tea used to be put up before modern packaging was invented. It is recommended by the People's Republic of China National Native Produce and Animal By-Product Import and Export Corp. as "A strong-tasting red tea, dark in color and good for the hangover, after drinking." Just break off a chunk, drop it in the teapot and cover with boiling water. Start with a small chunk: This tea turns the color of iodine in a very minutes and has a better, sour taste that is exactly - I checked this out - the same as the water that drips through a plant and comes out in the saucer on the bottom. If you like weird tastes, you may get to like pu erb tea, ever without a hangover. I did. I took a couple of pots.

KWANGTUNG MEDICAL teac comes in a huge paper pouch (25 cents) covered with all sorts of hortatory writing in both Chinese and English "Efficacious refrigerant is specially designed for the faculties of patients and travelers as a home preventative against disorders. But with two and half bowl of fresh water in moderate fire until volume of once bowl is left and drink when it is warm. One packet for each dose. Heavy ill two packets. Guarantee safe and harmless even for the pregnant women and the old men except when parturient. For the cases of: Catch Cold. Sore Throat. Indigestion. Constipation. Stop Thirst. Not to be taken when parturient."

Anyway, Kwantung tea looks like mixed pickling spices, smells musty and tastes like mulch. I did not get to like Kwantung tea.

There are also sweet Chinese drinks sold as teas that are not really tea at all. Cane and Imperane (99 cents) is pale beige beads that you can dissolve in hot or cold water. It is made of ground sugar cane and tastes fibrous and sweet, like sugary bamboo shoots. Chrysanthemum tea (80 cents) looks the same, dissolves as easily, is just as sweet, and has an odd, indescribable flavor that must be chrysanthemum - because that's what it's made of.