Who is this "Spider-man" you refer to? The Raimi Spider-man? The TASM reboot Spider-man who isn't even out? Including the latter would require creative involvement of another studio, including access to content for a movie yet to be released. Including the former would defeat the purpose for Sony.
There really is no way either side would ever agree to it. Marvel would, at best, gain an extra chunk of gross in exchange for losing creative control over their own franchise, as Sony could withdraw agreement whenever they wanted. And that is assuming Sony didn't insist on payment as well, which would defeat the point of the extra gross.
Meanwhile, Marvel would effectively be paying for free promotion of someone else's movie, presuming they used the TASM Spider-man.
And this is all presuming that including Spider-man would actually provide a significant increase in revenue, which is not at all certain. Yes, Spider-man is popular, but so is Iron Man. Are there actually a lot of Spider-man fans to be brought in, who'd not otherwise be seeing the movie for one of its other characters? There is no way to tell.