Cap or Stark HAD to die in Civil War since that would so how badly things have gotten
Really why does someone dying elevate the gravitas of the situation? The last fight between Cap and Stark says more for the characters than if either of them had died. Cap was within a hairs breath of killing Stark but his character stopped him from doing that, and had Cap not gotten in the way I believe that Stark would have killed Bucky. Struggling with having to kill a friend to get your revenge was more effective, seeing either of them die would have added nothing to it.
How that fact is handled in the next team up movie is more important, if they just come together and are all good buddies again then yeah poorly handled but if it proves to be a point of conflict even stretching in to the visions that Stark had in Age Of Ultron about how he killed his friends, then yeah nice.
As for the budget, well I imagine the actor payroll is going to be quite high for these two movies given the potential character list, fuck by that stage Marvel will have had just about ever actor in Hollywood on it's books so I would imagine that will cost a fair bit.