From what I've been reading, Boston is trying to low-ball the out of Atlanta right now. They don't even want to give up Jae Crowder for him. And all three of the Brooklyn picks are off the table. As I mentioned before, it seems like Horford's value is depressed enough to where the Spurs are going to be able to make a compe ive offer if they so choose.
That said, they shouldn't offer any starter for Horford. He's not an upgrade to the team if they just create another hole. That's even more true when you consider that Horford is probably not going to help them get over the hump against Golden State. You don't trade your best guard defender for a big when you're already going to have to play small. That just doesn't make sense. And the Spurs haven't been a team that trades production/chemistry for talent. Add in the fact that Horford would probably take too long to integrate, and I see this as an off-season move at best.