As much as Marvel would like Spidey under their control, I can't see the studio forking over $1 billion + for one character in the midst of a box office slump.
$1 billion is very justifiable if you can make it back within a movie or two. But I don't see such an offer happening, any Disney/Marvel/Sony deal would most likely be similar to the Disney/Lucasfilm/Paramount deal with Indiana Jones. Sony would keep the rights to previously made Spider-Man films, Disney would have full film rights to future Spider-Man films in return for Sony getting a certain cut of the box office.
Also, I wouldn't call 2014 to be a genuine slump. A slump would imply that we're going to see declines for years to come. When you compare this year to last year, the natural result is a massive decline, for this year only. Last year gave us hits like Frozen, the Hobbit: the Desolation of Smaug, Hunger Games: Catching Fire, Iron Man 3, Man of Steel, Despicable Me 2, Monsters University, Star Trek into Darkness, Thor: The Dark World, Fast and Furious 6, etc. This year we have no Pixar movie, very few childrens' animated films, no Fast and Furious movie, no major DC movie, and a collection of disappointing sequels in Transformers 4 and Amazing Spider-Man 2, lower box office returns are the natural result. 2015 however is going to bring us a new Fast and Furious movie, Avengers: Age of Ultron, Star Wars Episode VII, Inside Out, Ant-Man, Jurassic World, James Bond 24, the Good Dinosaur, Minions, and other films that have a lot of potential.
Though Spider-Man still has considerable box office appeal, it would take a decade or more for Marvel to earn back that investment in box office, physical media, streaming and TV rights. Not to mention the opportunity cost of postponing investment in a character (Captain Marvel? Black Panther?) already high up in the queue.
It would take far less than a decade for Marvel to earn back a $1 billion investment from Spider-Man, even without the film rights, Spider-Man alone (not counting the rest of the Marvel characters) is a $1 billion/year brand for the Walt Disney Company. It is one of the eight brands Disney has that make well over $1 billion/year to where it is on par with Star Wars, Disney Princesses, Cars, Monsters, Mickey Mouse, Winnie the Pooh, and Disney Jr.
Last year Dan Loeb, a leading Sony investor, proposed that Sony spin off their entertainment division and sell off 20% or so in an IPO. This is the kind of investment I can see Disney/Marvel making, with the Mouse buying anywhere from 20% to 51% of Sony Entertainment. Such a deal will give the Mouse some degree of control over Spidey, along with investment in franchises like MIB and Ghostbusters. It would provide Sony with a needed cash infusion and access to Disney's distribution network while maintaining a strong position in the entertainment industry.
That will absolutely not happen. Dan Loeb was shut down pretty hard over his Sony Entertainment spin-off proposal and Disney would never invest in a company like Sony Entertainment. Franchises like Men in Black and Ghostbusters really don't fit in with the Disney mold the way Marvel and Star Wars do; Disney already has a ridiculously strong position in the entertainment industry, they have three of the top five grossing films of the year; and their a merchandising behemoth. Really the only places were it would be more beneficial for Disney to expand in entertainment would probably be having a stronger position in the video games industry and music.