Queen Elizabeth's father used to speak of the British royal family as "the Firm" – a nickname that has stuck – and Kate Middleton definitely is good for business.

Even before she was Prince William's fiancée, the 28-year-old was well on her way to style-icon status, selling out a £40 (about $65) Topshop dress she wore on her 25th birthday.

"Kate is not a fashion icon yet but she may become one, so it is nice to see her wearing Topshop clothes," a spokesman said at the time. The dress sold out less than 24 hours after Kate was pictured wearing it

The $616 sapphire blue dress from Issa London Kate wore to the announcement of her engagement to Prince William also sold out within hours.

"Since the announcement of the royal engagement we have been inundated with requests regarding Issa," Harvey Nichols buying director Averyl Oates told Vogue UK. (Issa is also available in a selection of U.S. boutiques, and the dress sold out here, too.)

For those who couldn't get their hands on the dress – or couldn't afford to in the first place – British supermarket chain Tesco is offering a £16 (approximately $25) replica. The "Kate dress" is available online for overseas buyers from November 23.

Also on the heels of the royal wedding announcement, London designer Katherine Hooker – whose bespoke coats Kate has sported several times – announced plans to expand in the U.S.

The future queen's engagement ring – with its 18-carat sapphire surrounded by diamonds – has launched a wave of imitators.

Within 10 minutes of the release of photos of Kate wearing the ring last Tuesday, the website for New York's Natural Sapphire Co. crashed because of too many online orders.

"I called my wife and said, 'Honey, I'm not coming home tonight!'" Michael Arnstein, the company's chief executive, told the Associated Press. "We're in frenzy. This is changing our business overnight."

Arnstein's grandfather founded the company in 1939. Arnstein estimates the actual ring – which originally belonged to Princess Diana – is worth about a half a million dollars. (The sapphire alone is valued at $300,000).

Arnstein's company is producing $1,000 to $2,500 replicas using 1- or 2-carat sapphires surrounded by tiny diamonds, set in 18-carat white gold. Middleton's ring also is set in white gold.

"Congratulations to Prince William and Kate Middleton," reads the company's website

For those without access to a royal budget, on Saturday, November 20, jeweler Kenneth Jay Lane began hawking a $34.50 "Princess Simulated Sapphire Ring" on QVC. A version selling for about $50 has long been available on the British QVC, and it was selling nine times faster than usual within hours of the engagement announcement.

Prince William and Kate Middleton's wedding is expected to be a boon to the British hospitality industry.

"The wedding next year will mean that once again the eyes of the world will be turned to our nation," said Visit England chief executive James Berresford. "We will have a wonderful opportunity to showcase all that is best about the country."

Datamonitor's Verdict Research estimates the wedding could add nearly $1 billion to the U.K.'s economy.

Besides lifting tourism, a royal wedding could be blockbuster business for the souvenir business, which has been itching to get started down memento lane for years. (In 2006, Woolworths – which is now out of business – produced a souvenir plate that read "Celebrating the Royal marriage of William and Kate" with a question mark for the date. To see it, click here.)

Asda supermarkets – owned by Wal-Mart – immediately called printing firm Marvelpress to begin producing commemorative engagement mugs. The supermarket is also considering royal wedding-branded iPhone covers, champagne, and wine boxes.

It's not just retail giants who are getting in on the William and Kate show. Within hours of the engagement announcement, souvenirs – among them mugs, tea-towels, keyrings, and refrigerator magnets, some of them looking very homemade – began popping up on eBay. The site reported a 300 percent increase in William and Kate memorabilia within days.

Meanwhile, retailers who had no links to the royal wedding raced to cash in on Kate-mania. "20 percent off everything + royal wedding inspiration for Miss Middleton," read the subject line of an e-mail blast to subscribers from Coast, an English chain that sells affordable formal occasion wear (usually not more than $150) and frocks often used as bridesmaid dresses.