Jon Leuer makes about $11 million a year. Trevor Booker makes about $10 million.
Market for KO should be along the lines of $12-14 million per year, depending on whether some other team thinks he can start.
As for the KO / Horford pairing:
Per nbawowy!, when they share the floor the Celts have a 117 Offensive Rating and a 113 Defensive Rating.
Those numbers without IT on the floor: 103 Offensive Rating, 107 Defensive Rating.
It's only a 170 possession sample size, however.
The numbers for the starting lineup with Kelly are pretty great, though:
128 Offensive Rating, 107 Defensive Rating (122 possession sample)