Boston's Chinatown isn't large, and it doesn't have the consistently high-quality food you can find in San Francisco or Toronto Chinatown, but if you look you can find places that serve Chinese food that's both good and cheap--King Fung Garden is one of those places.
Don't be put off by the divey exterior. Yes, the flourecent lighting, formica booths (remnants of a former life as a pizza parlor), and space heaters don't look so hot, but the peking duck is arguably the best in town. Call 24 hours in advance to order it, and you'll recieve a three course feast--duck breast and crispy mahogany-colored skin with pancakes, duck stirfry, and duck soup--that's ample enough to feed four, all for around $30. You'll realize why dining luminaries like Emeril visit King Fung Garden when they're in town.
Their other northern Chinese specialties are good, too, from the crunchy fried scallion pancake drizzled with toasted sesame oil, to the coal-fired Mongolian Fire Pot.
Service is cheerful and efficient. If you're confused about what to order, the friendly owners will be happy to recommend dishes for you. If you go in the winter, though, bring a nice, warm coat since the small dining area tends to be drafty.