Appearing last week on "The Daily Show," "Iron Man 3" star Robert Downey Jr. was coy when asked whether he will "keep going" with Marvel's flagship franchise, responding simply, "I don't know." "I had a long contract with them," he added with an exaggerated wink to host Jon Stewart, "Now we're going to renegotiate."

What seemed at the time like a lighthearted exchange, another instance of Downey being Downey, now appears to be the first clear sign of financial tensions behind the scenes of the Marvel Cinematic Universe.

Deadline reports that Downey, who made more than $50 million from "The Avengers" alone and may have already earned $35 million from "Iron Man 3," isn't locked in to Joss Whedon's "Avengers 2." The Hollywood Reporter adds that although negotiations have begun for the sequel to the $1.5 billion blockbuster, and "The Avengers 3" beyond that, "Iron Man 4" hasn't been part of the discussions. That may lend credence to suggestions that Downey isn't interested in returning for another solo film.

However, he's not the only one angling for a bigger payday. Notoriously frugal Marvel signed stars to unprecedented multi-picture deals -- Samuel L. Jackson, for instance, initially agreed to nine -- low-balling some actors while offering significantly more generous terms to others. (Scarlett Johansson is said have been offered just $250,000 initially for "Iron Man 2," a figure renegotiated to more than $400,000.) Deadline contends Downey's "Avengers" co-stars Jackson, Johansson, Chris Evans, Chris Hemsworth, Mark Ruffalo and Jeremy Renner are now "counting on at least $5 million upfront and better back ends for 'Avengers 2.'"

Downey Jr.'s 'sly' wink to Jon Stewart

While Evans is believed to have made his deal for "The Avengers 2" when he signed on to "Captain America: The Winter Soldier," the others don't possess the leverage that Downey does: The Hollywood Reporter notes that "Iron Man 2" outgrossed 2011's "Thor" by 39 percent and "Captain America: The First Avengers" by a whopping 69 percent worldwide -- and "Iron Man 3" has already grossed more than $680 million worldwide.

What's more, Marvel isn't shy about recasting roles. When Terrence Howard wanted more money to return as James Rhodes in "Iron Man 2," the studio simply replaced him with Don Cheadle. And Ruffalo is the third person to play Bruce Banner on the big screen. If more evidence is required, THR offers contention that Marvel used threats of recasting during negotiations with Evans and Gwyneth Paltrow. The message is clear: Most of the players in the Marvel Cinematic Universe are replaceable, like detectives on "Law & Order."

But Downey? His departure would be financial blow to the studio even bigger than the star's cut of receipts, with analyst Doug Cruetz telling THR it could cost Marvel as much as 9 percent of its near-term earnings. "It would be a definite negative for that particular franchise. He is Tony Stark," he said. The other individual franchises -- Thor, Captain America, Hulk, etc. -- don't have near the level of box-office potential that Iron Man does. The other way to look at it is that Iron Man would probably look more like those other franchises in terms of box-office performance without Downey."