Ellen Ruppel Shell has been studying the price, the value and the cost of things — all sorts of things, bookcases and summer dresses and plastic wrap and key chains and tube socks and hammers. And shrimp, the subject of a brief, appalling tale that illustrates what she’s up to in this important book. Until the 1970s or so, shrimp was a luxury for most Americans. Thanks to a huge boom in shrimp farming, especially in Thailand, supplies soared during the ’80s, and prices started to fall. Now, Ruppel Shell reports in “Cheap: The High Cost of Discount Culture,” we eat shrimp as casually as we eat tuna fish — more casually, in fact, since our consumption of shrimp actually outstrips our consumption of canned tuna. If you order shrimp at a midrange restaurant like Red Lobster, you get a hefty portion for about $16 — and if you’re there during the annual “Endless Shrimp” promotion, you can keep on ordering until you topple right over into the garlic butter. It’s still only $16...