Truth is, pretty much none of the starting five have shown that they should be kept if something better comes along. I would say that Ross has shown the biggest one year improvement, since he was so shit last year. If he improves as much next year he is a keeper. JV has improved, and if he shows the same level of improvement next year, he is a keeper. Lowry...nuff said. With DeMar, it's consistency. He has had spells of 6-8 games where he has looked like a world beater, then he regresses. Amir seems the most consistent, barring the previous stretch of games when he was dealing with something. And his numbers are adequate for a starting 4.
Maybe you look to seriously upgrade the bench...get the guys at the 6th, 7th and 8th spots in the rotation to be significantly better. Even in the ano team can play the starting 5 for 48 minutes for 7 games. It might be easier and cheaper to upgrade the next 3 or 4 spots on the bench. Instead of giving max money to a couple of starters, spread that 10-12 million/year shoring up the bench positions.