Mills is going to command at least $10 million pretty easily next season, whether as on the MLE or with the Spurs. I think it's 50/50 that he's gone. The Spurs have a bunch of young guards already on roster and they can easily redistribute his minutes, though his shot making will be missed.
I think Gay's gone, Aldridge too, and DeMar is almost certainly gone. He'll likely command around what he's making now, but over at least 3 years. That's a lot to commit, and could tie-up future cap, especially with future extensions to Walker and KJ. As good as he's been, DeMar's production could probably be redistributed between Murray, White, KJ, and Walker.
I think the Spurs will definitely try to get Collins or Lauri but will probably strike-out. Probably a patch-work at PF with Lyles returning for depth, Luka getting backup minutes and a big question mark for the SL unless they go small again.