I think Curry's market value is around $4 million, but if need be, I think the Kings should overpay a couple million extra to keep him because he doesn't just plug one hole, he plugs several. He can shoot, pass, play team ball, play SG, play PG (not full time), play defense, come off the bench, or start. He can wear several hats and that's what tip it in his favor for him. Furthermore, he's consistent and he has no drama, he is basically the perfect role player (aside from his size). I'd go further and say that the Kings had not had a SG this good after Tyreke, which says more about the awful SGs before him but still.
From a roster point of view, he is versatile and allows the GM to build the team in several ways (get a starting quality SG and make him come off the bench. Get a strong play making SG and partner with Curry, etc, etc). He is one of those guys who can fit several ways, again, the versatility thing is big for me.
Using a Kangz perspective, I just know that if the Kings let him go, they are going to get rejected left and right from better SG FA. The last thing I want is a repeat of the Belinelli signing where we got an average player for good money only to find out he is below average in a Kings uni. When I look at the free agent list, it's obvious to me that the stud SGs we have no chance of getting and the tier below that are no better than Curry (yes debatable but that's mo).
I say live with the devil you know and the grass not always greener on the other side.