At first glance, Tsukiji is just another luncheonette converted into a sushi bar, with fish that is no fresher than average and cooked dishes that are no better than ordinary. But two characteristics set it above others of its type: first, the prices. Sushi is $2 an order and even rolled sushi (maki) are $2. An order of sashimi is only $3. That makes it as least as cheap to order ,a la carte as in an assortment. The second outstanding characteristic is the personnel, especially the owner and the sushi chef who communicate well, explaining dishes that defy description. It makes a big difference when you don't know the subtleties of three kinds of clams or the possible combinations of fish and seasonings that might be made into seaweed- wrapped rolls. Most main dishes are about $7.50 to $8.50 and include a easonably light and crisp but not noteworthy tempura, grilled swordfish that was perfectly fresh but a bit chewy, several teriyakis and a couple of pork dishes -- hardly different from dozens of other Japanese menus. The point of Tsukiji is the sushi bar, where sushi and sashimi, expertly prepared, are a good value. 785 Rockville Pike, Rockville. 294-9160. L $5.50-$7.50 D $4.50- $12. L daily, D daily. Closed Sun. MC, V. Res sugg. Full bar.