New York ? It’s only a trailer sitting in the middle of Times Square, but the TKTS booth offering same-day discount theater tickets has sold tens of millions of them since it opened 30 years ago.

It’s been open 365 days a year since June 25, 1973.

Back then, the average TKTS ticket went for $4.50, at 50 percent off. Now, it’s ten times higher, the average price being $40.75.

Finding the 10 windows of the TKTS booth is easy: There are usually two long lines winding around it at Broadway and 47th Street, including New Yorkers and visitors from around the world.

Last year, a new TKTS location was set up at the South Street Seaport in lower Manhattan, replacing the booth at 2 World Trade Center destroyed in the Sept. 11, 2001, terrorist attacks.

Both locations, operated by the nonprofit Theatre Development Fund, have collected $940 million in revenue during the three decades, selling a total of 43 million tickets to Broadway and Off-Broadway spectacles, as well as music and dance events such as the American Ballet Theatre and the New York City Opera.

Discounts range from 25 percent to 50 percent off, with most tickets going for half-price.

On Tuesday, four women from Texas arrived armed — with umbrellas they had brought for New York’s record June rainfall that they turned into sunshields as they waited on the 91 degree, sun-drenched pavement.

“I’m not used to standing in the sun! We have roofs over our heads in Texas when it’s hot,” said Martha Gonzalez, a retired kindergarten teacher from Premont, Texas.

How long had they waited? “Half an hour. But it feels like five hours,” said Alma Salazar, a retired school official, also from Premont.

The goal of TKTS has been to make reasonably priced tickets available to new and younger audiences, while helping shows stay open longer. When a show doesn’t sell out, the producer or theater manager, working with the box office treasurer, makes the decision to offer the empty seats through TKTS.

“It’s done exactly what it set out to do,” said Victoria Bailey, executive director of the Theatre Development Fund. “First, people can buy tickets who otherwise couldn’t afford to. We also get large numbers of first-time theatergoers who come back again and again.”

With TKTS as a model, discount ticket booths also have been set up in Boston, Washington, Chicago, Denver, Pittsburgh, Philadelphia, San Francisco, Toronto, London and Sydney, Australia.

Paying either cash or with a traveler’s check — all that’s accepted — theatergoers wait in line anywhere from five to 90 minutes, choosing from as many as 50 shows, said TKTS spokesman David LeShay.

But forget about discount tickets for “hot” shows like “The Lion King,” “The Producers” or “Hairspray” — they’re sold out.