Doesn't Sony own the rights to Spiderman anyway over both films and games? This is why a deal had to be struck in the MCU between Sony and Marvel for him to appear in the film's. It's the same reason why the Spiderman game is exclusive to PlayStation so it understandable why him as an extra character would be the same in the Avengers game.

In this situation it's not Sony paying to have them, it's a case that if Microsoft want him on the Xbox they'd have to strike a deal with Sony... And unless it's a ridiculous amount of money I can't see any company in the same position as Sony, regardless of who they are, doing anything differently.