One couple might have snagged a package deal with airfare thrown in. Another likely caught a special hotel promotion, or found a last-minute bargain or lucked into a complimentary perk. Any of those circumstances can make a room less expensive than any reserved by telephone or Internet. Rates spin up and down nearly as fast as the wheels on a slot machine.

Behind a discreet door near the Mirage Hotel Casino reception desk near a lobby that might be described as Kon-Tiki-meets-Trader-Vic's, management toils in a suite of no-nonsense offices and cubicles.

Franz Kallao, 30-something vice president of hotel operations at Mirage, told me people in the lodging business refer to the callers and browsers of hotel Web sites as "transients," people content to accept what's known as the "rack rate" when they book a room.

"We employ a yield-rate management program for what we call our transient selling," Kallao said. "Those are known as our 'selling rates,' and those are based on availability, so as the supply is larger, the rates will be lower, and as supply diminishes, rates will be raised."

This may sound like Economics 101, but really it's Economics 101 on speed. Sophisticated software programs show people like Kallao how each day is shaping up and let them raise or lower rates accordingly.

When the preferred arrival dates are less than a month away, those prices might change every few hours. One day in late September, for example, I checked on the cost of a "deluxe" room at the Mirage — the lowest-priced unit — for Oct. 24. It was selling for $209, plus 9 percent tax. When I checked a day later, the price had gone up 50 bucks.

"The software program, basically, shows us pacing, how a particular day is pacing," Kallao said. "Our goal every day is to reach 100 percent occupancy. And we try to do that at the very highest rate possible."

Evidently, Oct. 24, 2004, was pacing well.

Certain pricing principles prevail all over town.

• Summer rates tend to be lower, even though in the past few years summer visitors have been streaming into town in greater numbers. A lot of the summertime crowd, however, includes vacationers, presumably with tighter budgets than the conventioneers who prefer to gather in great numbers during the cooler months.

A stay in the cheapest Mirage room from July 10 through 12 in 2005, for instance, is projected to cost $129 a night, plus tax.

• All things being equal, a room on the weekend will cost more than a room on a weekday — which includes Sunday.

For instance, when the International Consumer Electronics Show comes to town Thursday, Jan. 6, 2005 — bringing 115,000 show-goers — the same room that went for $129 on weekdays in July is slated to cost $299 the first night and $359 a night over the next two nights. On Sunday, Jan. 9, it goes back down to $299. Supply and demand.

• Prices typically rise during long holiday weekends and on holidays. Huge conventions or major events also push rates up.

On Christmas Eve and Christmas Day, the daily rate on that same room is $189.

• A slack period might engender a promotional rate. Recently, Mirage offered Dec. 19 through 23 for $69 a night, whereas the rack rate on the hotel Web site was $89 a night for the same time span. The low prices reflect the fact that visitation goes way down in the week or so just before a big holiday. There are few, if any, conventions then, and people are gearing up for the Yule, which, come Christmas Eve, might involve waiting for Santa in that now-$189 room.

All prices mentioned were on the Mirage Web site as of the last two days in September. The special Dec. 19-23 offer expired Sept. 30, but no doubt, something else equally tempting will replace it.

People also may check rates far into the future. A lot of hotel Web sites quote prices six or more months in advance, and those vary wildly, too, although they don't change from day to day. A long way down the road, charges are based on past performance. The computer knows how many people booked on that date in years past and what big conventions and holidays will be coming up.

Hotel executives also play hunches, drawing on their own experience. "We don't allow a program or formula or system to completely dictate to us what the rate should be for a particular day," said Kallao. "There are a lot of different things the computer program might not be aware of — what the competition is doing, holidays, conventions, special events. If there's going to be a prize fight, as there has been for Mexican Independence Day for many years, that's a big weekend."