Audiences across the world watching Meghan Markle marry Prince Harry on Saturday are expecting romance and pageantry, including diamond-studded tiaras and 21-foot-long dress trains. But such memorable opulence doesn't come cheap. While the average U.K. wedding costs about $37,000 (£27,000), a considerable figure in and of itself, England-based luxury wedding planner Aimee Dunne tells CNBC Make It she estimates that, before security, the wedding of Prince Harry and Meghan Markle will cost $2.7 million (£2 million). A whopping $40.1 million (£30 million) more will be dished out on keeping the betrothed and their guests safe, reports U.K. wedding site Bridebook. Here's how the expenses break down.

Food and beverage: $680,000

Head chef Mark Flanagan probably won't be serving ortolans on the big day, but there will be no shortage of classy hors d'œuvre and desserts, including a much-anticipated lemon elderflower cake. With 600 guests attending both the lunch and dinner receptions, plus 2,640 guests from the public who will be provided with tea and a hot snack on the grounds of Windsor Castle, Dunne expects the food and beverage to amount to a quarter of the total budget. "There will be lots of champagne and whiskey, I'm sure, for Harry," she says.

Glass marquee: $400,000

While the prince and soon-to-be duchess don't have to pay to rent out St. George's Chapel or the Great Hall, there are rumors that the royals are building a glass marquee for the reception. The construction and delivery could cost as much as $400,000, according to Dunne.

Dress: $270,000

The latest front-runner in the question of what Markle will wear on Saturday is a dress from Ralph & Russo, London haute couture designers which have previously prepared outfits for stars such as Gwyneth Paltrow, Angelina Jolie and Beyonce. Ralph & Russo supplied Markle with the dress she wore during her official engagement photo shoot. ENGAGEMENT PHOTO The Telegraph reports that the dress will cost in the ballpark of $135,000, but Dunne expects it to be as much as $270,000. That's standard for royal weddings. It is rumored that Harry will wear something more "low-key." Markle is also expected to change her outfit in the evening. Kate Middleton, the Duchess of Cambridge, wore an ivory satin gown with a diamante belt and matching bolero after her reception, reports The New York Times.

Flowers: $130,000

Given the size and scale of the wedding, hiring a team of florists to decorate Windsor Castle will be costly. "It will be tasteful. I don't think it's going to be overly kind of glam and wasteful in terms of what they spend," says Dunne, "but even in that case ... they could easily spend £100,000 on flowers."

Prince Harry and Meghan Markle attend an official photocall to announce their engagement at The Sunken Gardens, Kensington Palace on November 27, 2017 in London, England Max Mumby/Indigo | Getty Images

The rest of the budget

Other considerable expenses include entertainment. "The music will be under the direction of James Vivian, director of music, St. George's Chapel, including the Choir of St. George's Chapel, and a selection of choral groups, soloists and musicians," Kensington Palace announced. MUSIC TWEET So far, Karen Gibson and The Kingdom Choir and 19-year-old cellist Sheku Kanneh-Mason are confirmed performers for the wedding service. There are also rumors circulating that stars such as Elton John, the Spice Girls, Ed Sheeran and Adele could perform, reports GQ. As for the couple's wedding rings, the bands are reportedly made out of their Welsh gold, reports Bridebook, and there is a chance that Prince Harry follows in his brother William's footsteps and doesn't wear one at all. Markle's engagement ring was designed by Harry himself using a central diamond from Botswana and two flanking diamonds that belonged to his late mother, Diana, Princess of Wales. Experts estimate the ring could be worth as much as $350,000. Finally, the cost of logistics can add up, says Dunne, especially for a high-profile wedding where many preparations are made under wraps. "Keeping things quiet — that has a cost associated with it," she says, because secrecy tends to require extra people and more time.

Who's paying?