Four of Long Island’s most frightening seasonal attractions are coming together for the first time this Halloween in hopes of bringing scares to bigger crowds.

Owners of Chamber of Horrors NY in Hauppauge, Darkside Haunted House in Wading River, Gateway’s Haunted Playhouse in Bellport, and the Haunted Mansion of Melville at Schmitt’s Family Farm are teaming up this year and offering discounted rates for chill-seekers. The partnership — billed as an “unholy union” — allows the businesses to pool advertising efforts and potentially attract more customers.

“It’s going to be a lot more cost-effective, and we’ll be able to cover a much larger area,” said Matt Guiliano, a former pro baseball player with the Long Island Ducks and part owner of Chamber of Horrors.

As part of the monster merger, the four will be advertising as one group through print, billboards, radio and online.

Haunted houses, which feature elaborately designed rooms staffed with live actors dressed as everything from flesh-eating zombies to bloodstained serial killers, are big business, industry observers said.

Annual ticket sales are more than $1 billion, according to estimates from the Haunted House Association, a St. Louis-based industry group.

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Behind the local haunts’ cooperative effort is Scream LI.com, a website launched this summer, which will serve as the one-stop shop for ticket and event information. There, customers can buy individual house tickets — around $25 — or spring for discounted packages for all four attractions for $80.

Guiliano, who has hosted Chamber of Horrors at his indoor baseball training facility Play Like a Pro for the past three years, said setting up a haunted house is a costly endeavor, and that working cooperatively with Island competitors made sense. “There’s so many options for us to flood the market” with shared promotions, he said.

To keep their haunted houses enticing to customers, owners have to stay up to date on all that’s new and most horrifying, leading many to revamp their layouts, themes and props each year. Some, like Guiliano, who uses about half of his 33,000-square-foot baseball facility for his haunted house, have to rebuild the indoor structure each year.

Planning can start as early as Nov. 1. “It’s like Halloween never ends,” said Guiliano, whose haunt saw about 10,000 customers last season.

Operators said it’s fairly common to spend $125,000 or more just to get a house ready for the season, especially when the rising costs of labor, insurance and advertising — which can run up to $15,000 alone — are included.

Bill Schmitt, co-owner of the Haunted Mansion of Melville, who’s been running his seasonal haunted barn and accompanying 1-acre corn maze for about 22 years, said that updating your attraction is a must.

“You have basic rooms that you set up, but you’re always looking for something new,” Schmitt said.

Each year, owners and operators of all four local haunts head to St. Louis for TransWorld’s Halloween & Attractions Show, a trade expo for the haunted house industry. Schmitt said operators can “spend $20,000 . . . in a heartbeat” at the March show.

Mike Meola, owner of Darkside Haunted House in Wading River, an 18-year-old haunt, agreed the costs of running a scare-traction have risen. “You used to be able to get a body for 200 bucks,” he said. “Now, a body is $750.”

Given that many of the props, such as realistic corpses, are produced by small special-effects companies and artists, prices can easily hit a couple thousand dollars per item. All those costs make it a hard industry to break into, experts said.

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“Haunted houses are so ridiculously expensive to open now,” said Larry Kirchner, editor of HauntWorld Magazine and spokesman for the Haunted House Association. “It’s not just some automatic moneymaker.”

Paul Allan, manager of Gateway’s Haunted Playhouse, which started in 2009 and operates out of the Gateway Playhouse theater, said that, for him, opening a haunt has provided the business with a new ticket driver.

“Because we’re set up and we have year-round theater expenses, this haunted house has just become another production for us,” Allan said. “The only way to keep audiences coming is to offer different things.”

The ScreamLI partners said they anticipate that their efforts to work together will help them control spending and reach an audience beyond Nassau and Suffolk counties.

“It’s smart business to work with your competition,” Meola said. “With the economy the way it is, you’ve got to change how you do business.”