Of all the notes I get from readers about wine issues, one topic by far generates the greatest irritation — and it isn’t about the prices wineries charge.

It’s about the prices of wine in restaurants, which are usually significantly more than in retail shops. And the practice is getting worse.

The major grievance seems not to be about rising retail prices, but about markups on ordinary wines in restaurants.

My reply to most readers is that a huge reason for this is not only the prices of the wines themselves, but restaurateurs’ greed (marking up wines outrageously) as well as two ancillary “hidden” costs that few people take into account:

The sales tax and the tip. The latter is insidious and often a forgotten factor.

When dining out in the United States, we tip the wait staff. A 10 percent to 15 percent tip once was normal, but lately 20 percent is seen as appropriate.

Tipping remains an American issue. It is not widely practiced around the world. In many countries, a service charge is added to the tab, so that “tip” is out of diners’ control.

To be sure, tips are appropriate in high-end restaurants, where sommeliers perform crucial tasks. The sommelier is a noble calling that requires many skills. They not only remove old, fragile corks with aplomb, but also ascertain whether the wine is sound.

Some “somms” are even expert enough to determine if a wine is authentic. Considering all the stories lately about fraudulent wine, a pro can be a great asset in a quality restaurant.

Many waiters think of tips as part of their wage. Even if the hourly wage isn’t great, tips are a bonus. But problems occur with tipping in the real world. Not all servers benefit.

— Tip-sharing in some restaurants is an attempt to equalize the tips all servers receive. Result: many great servers get less than they deserve and poor ones benefit.

— Diners often are reluctant to tip 20 percent, arguing that’s too much for poor service. Many diners leave less than 15 percent.

— Most diners I speak with are critical of high markups for ordinary wines. Result: average tips are declining. (Result: servers suffer, not owners.)

Also, we have seen increases in restaurant wine profit margins in the last decade, despite of the fact that we have still not yet fully recovered from the 2007-08 recession. Many restaurants act as if the recession never happened. What’s afoot now is a rejection by many consumers over what they see as a ripoff.

One savvy diner told me last week, “A few years ago, restaurants had decent Sauvignon Blancs for $30 a bottle. Now they’re $40!”

And here’s the rub: The tip of 20 percent. (Reality: on that $40 bottle, you’d be paying someone $8 to open a screwcap!? How much expertise was needed there?).

Add in 9 percent more for the tax and that “$40 Sauvignon Blanc” is nearly $59! In some cases, that’s equal to the cost of three main dishes!

No wonder I get so many complaints.

Case in point: A very nice New Zealand Sauvignon Blanc is the 2017 Chasing Venus. Restaurants typically pay less than $10 a bottle to buy it and put it on their wine lists. Retailers around the country carry it for $15 or less.

If you were to dine at the upscale Roof on Wilshire restaurant in Los Angles and ordered that wine, after the tax and tip you would pay $62 to drink it.

It’s no wonder restaurants in every major city are saying dining is down. Anecdotally, a rebellion against high restaurant wine pricing is afoot.

Wine Tip of the Week: Consider fighting back: The next time you see excessive markups for wine in a restaurant, tell the manager that you’re a wine lover who doesn’t appreciate seeing prices that are three-times the retail price. Then order no wine that evening. And consider boycotting places that practice such legal thievery.

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Dan Berger lives in Sonoma County, where he publishes “Vintage Experiences,” a subscription-only weekly wine newsletter. Write to him at winenut@gmail.com. He is also co-host of California Wine Country with Steve Jaxon on KSRO Radio, 1350 AM.