As planes fill up, procrastinators face higher prices and undesirable flights, such as a red-eye or a connection in a snowy city prone to delays.

6:59PM EDT October 20. 2012 - PHOENIX -- Brandy Reinke plans to spend Christmas in Denver with her family and might visit over Thanksgiving, too, but she hasn't even begun to shop for airline tickets.

"Smart people buy them in the summer, but I don't get that far ahead of the eight ball," the Phoenix woman said.

Reinke probably shouldn't wait much longer. There's no magic formula for when to buy tickets for holiday travel, but with the first wave of the airport rush less than a month away, crunch time is rapidly approaching. As planes fill up, procrastinators face higher prices and undesirable flights, such as a red-eye or a connection in a snowy city prone to delays.

Rick Seaney, CEO of travel website FareCompare.com, already has started his last-minute meter for Thanksgiving travel. Through Halloween, travelers can expect to pay $2 more per day per ticket, he said. In November, that figure will jump to $5 per day.

"You can calculate what your procrastination price will be," he said.

Travelers headed out for Christmas have a few more weeks to shop, Seaney said, unless they plan to fly on the most popular days, such as the weekend before the holiday. Christmas is on a Tuesday this year.

"If you're going on the super-peak times, you might as well lock in now and get non-stop (flights) at a perfect time," he said.

Airlines charge 30% to 70% more for holiday travel, depending on routes and dates, Seaney said. The lower end of the range is for off-peak days such as Thanksgiving and Christmas Day and, in December, any day before Dec. 19. After that date, expect to pay at the higher end.

"That's the day when airlines start treating everything as more expensive," he said.

Holiday airfares are higher this year because ticket prices in general have been on the rise. Major airlines have raised fares seven times this year, most recently a week ago. That is on top of nine increases in 2011. Through September, average fares were up 4.1 percent for domestic flights, according to figures released last week by Airlines for America, the industry trade group

For Thanksgiving flights, Fly.com says fares on some routes are up 15 percent from a year ago nationally.

Airlines have been able to boost fares because travel demand is strong and they have reduced the number of seats over the past few years in a bid to improve profits in the notoriously up-and-down industry.

Lindsay Hansen was surprised by the high fares and reduced flight options between Phoenix and Houston when she started shopping for Thanksgiving tickets a couple of weeks ago. The cheapest round-trip fare she and her husband initially found was more than $400 a person. They have two young kids, bringing the bill to $1,200 before baggage fees and a rental car.

They broadened their travel dates in hopes of finding lower fares. Last week, they found a $385 flight on US Airways during Thanksgiving week. Hansen recalls paying between $250 and $275 per ticket to visit her in-laws in Houston over Thanksgiving a couple of years ago.

They were on the fence about spending that much money but by the end of the week had booked the tickets because they were worried the fares would go higher.

"We bit the bullet," Hansen said.

Fares aren't the only costs on the rise. Some fees also have gone up since last holiday season.

Travelers also should know that many major airlines, including Tempe-based US Airways, are reserving more seats for passengers willing to pay extra to sit in certain spots, usually window and aisle seats up front. Passengers who want a guarantee they can sit next to friends or relatives may have to pay up if they book last minute and the "free'' seats are gone.