"It would be easy for them to become the next billion-dollar brand," says Tina Wells, C.E.O. of Buzz Marketing Group, a youth marketing agency and trend consultancy in New Jersey. "Touring is probably most profitable for them now, but we haven't begun to see the full potential of things. They're stylish: if they did a clothing line, it would make sense."

Wells warns that the boys would do well to stick with Disney, an adept marketing machine for its stars. Others who left to pursue a more mature image elsewhere have seen their careers flounder. Lindsay Lohan is a case in point.

"Lindsay Lohan had a Disney deal making her $7 million a movie," Wells said. "Who in the world has paid her anything close to that since?"

The Jonases are likely to stick with Disney for the near future, since their Disney Channel show and movie have yet to launch. But it's an open question whether those projects will be anywhere near as successful as Cyrus's, whose 3-D tour movie has grossed $70 million worldwide so far and whose show, Hannah Montana, averages 3 million viewers an episode.

"There's no doubt that they're incredibly successful, that they are incredibly versatile, and that they have the stamp of approval from parents," says Michael Wood, vice president of TRU, a market-research firm in Northbrook, Illinois, of the Jonas Brothers. "But I think it is more of a stretch in terms of playing in all of those categories."

If you're Miley Cyrus, Wood says, your fans want to be you—they will buy your records, wear your clothes, and buy anything branded with you, from perfume to chocolate.

"It's very different for the Jonas Brothers," Wood adds. "Are young guys going to want to dress like them? I'm not so sure. I think gender alone will be somewhat of a challenge."

In the end, the surest bet for the brothers may be to face the music—literally.

"Miley Cyrus was built by a TV show," says Bob Lefsetz of the Lefsetz Letter, a music industry newsletter. He sees the Jonas Brothers as a more authentic, and therefore more sustainable, phenomenon.

"They had a deal with Sony before Hollywood Records. They play their own instruments, have a hand in writing their songs," Lefsetz says.

Because of that, the group might expect "a longer sales arc with less high peaks." Lefsetz agrees the act has major earning potential, but won't speculate as to how much.

With such uncertainty about how the brothers should leverage their fame, it's not surprising that no one is willing to precisely forecast their potential earnings. But the brothers themselves are doing everything they can to keep the future looking bright.

"They understand how to take care of their partners," says Cory Isaacson, a partner at Walton Isaacson, the marketing and branding firm who brokered a Burger King sponsorship for the brothers' current tour.

"The boys said one time that they know they're on top but they're living like they're on the bottom. They're working around the clock, and everybody wants a piece."