“I have to admit we do take a little bit of an advantage on a markup,” says Gillian Ballance, wine director for the PlumpJack Group, which owns many San Francisco Bay Area restaurants, including Jack Falstaff, Balboa Cafe, and Farm. But she justifies the price increase by saying it’s “the expense of having the bottle open just for one person to have a glass.”

David Rosoff, managing partner in LA’s Pizzeria Mozza and Osteria Mozza, explains, “Wine by the glass is not formatted to be a ripoff, but it’s generally a lower cost percentage than you run by the bottle. You try to make money by the glass to support your bottles.” The cost percentage is figured by dividing the cost of the menu item by the total sales of that item. At a typical cost percentage of 20 percent for wine by the glass (and a judicious five-ounce pour), a restaurant pays for the whole bottle when one customer buys a glass.

It sure sounds like a ripoff. But let’s look at it from the restaurateur’s perspective for a second. A lot of work goes into a wine-by-the-glass program at a restaurant like Mozza. Rosoff works with his wine buyers to find good deals on wines that are pleasing solo but also pair with a variety of dishes on the menu. A glass of wine, says Ballance, is also the opportunity for someone to try something he’s never heard of for 10 bucks instead of investing $25 for the bottle at retail. “You also have to assume in the glass price the inevitable wastage that occurs for keeping bottles open: spoilage (if the entire bottle doesn’t get poured in one or two nights), tastes that people request, glassware, and service.”

Compare wine to liquor or beer. Rosoff says, “People pay through the nose for a cocktail. They know what the vodka costs in a Cosmopolitan, but [they’ll] pay $12 for it.” Ballance adds that the cost of a pint of beer is probably about 50 cents to the restaurant, but many will sell it for $6, a markup that makes wine seem like a bargain.

Restaurants will often let guests taste a wine or two before ordering a glass, and, Ballance says, “trying a couple before buying makes that $10 glass of wine more worthwhile.” And she also suggests that you ask about the freshness of the open bottles. “In fact,” she notes, “when I go to a restaurant and I like all of the by-the-glass selections, I just ask them to pour me whichever bottle was opened most recently.” If she doesn’t know the brands being offered on the list, her go-to wines by the glass are Malbec from Argentina, Sauvignon Blanc from New Zealand, and Côtes du Rhône from France: “The quality in all those wines is uniformly pretty high. For $9 or $10 it’s hard to go wrong.”

You could save money by buying a bottle of wine and taking the remainder home (although this might tempt you to drink more than a glass). Some restaurants also offer a variety of half bottles. But sometimes a glass of wine is an enjoyable little luxury, and I’m not going to ask my wife to give up her occasional glass of $20 Champagne—not yet.