ST. PETERSBURG — Sports fans may need to take a tip from airline travelers: Buy your tickets early, before prices go up.The Tampa Bay Rays have made what they consider a minor adjustment to ticket prices, but one effect is that the cost of some tickets could go up during the year. And the Rays aren't alone. The Tampa Bay Lightning began a similar practice earlier this year, and its ticket prices now fluctuate slightly according to demand.Under the old Rays system, a fan would pay more to watch a game against the popular New York Yankees than to see a struggling club such as the Houston Astros. The most desirable games were called "diamond" games, which cost more than "platinum" games, and those cost more than ones rated as "gold" and "silver."The team has stopped using the labels, and now says it could change prices during the season based on team popularity or a special event at the stadium."It's really very similar to the system before," Rays spokesman Rick Vaughn said. "We just added some flexibility, and we're moving away from using those names."We've always encouraged our fans to buy in advance," and this system reinforces that, he said.Vaughn used an example: Say you buy a ticket today for a Friday night game in June. But then later, a band is scheduled to play a concert after that game. Vaughn said the Rays might increase the ticket price, because fans will be getting something extra.If you bought your ticket early, you paid less. If you wait until later, you could pay more.Now, imagine the Rays in a pennant race, playing one of their rivals in a crucial end-of-season game. Will the price of that ticket go up, too?"It's possible," Vaughn said.At Tampa Bay Lightning games, the price can go up or down a few percentage points — possibly as much as 10 percent — as the game approaches, said team spokesman Bill Wickett.It's a system known as dynamic pricing, he said, and has become increasingly common at sports venues across the country.Of a half-dozen Rays fans the Times interviewed outside Tropicana Field Thursday, most weren't thrilled with the idea."I think they should set the price at the beginning of the year, and it's what the price should be for the whole year," said Ed Hunter, 79, of St. Petersburg, a retired postal employee."I don't think it's fair," said Karen Moyer, 50, of St. Petersburg. But Beth Cadwell, 29, of St. Petersburg could see some benefits of the system, because now fans know that buying early might lock in a better price.Although some don't like the change, everyone interviewed Thursday said that overall, Rays ticket prices are reasonable. Cadwell said she can often get into Tropicana Field for $10 to $15 on a weeknight, and "that's a great price to see a Major League team that I love."Although Vaughn said some Rays tickets could go up during the year, he didn't expect any to go down. He pointed out that ESPN in 2012 ranked the Rays as the No. 1 team for "affordability," out of 122 Major League, NBA, NFL and NHL teams. The Lightning was No. 14 on the same list."We feel our tickets are priced well to begin with," Vaughn said.