If I had to go after a SF who could be gotten 'cheap' who is playing in Orlando, I would probably go for Austin Daye.
He's stuck behind Prince and isn't getting any time at the SF spot, and now they want him to start trying to play the PF to get time, but he doesn't have anywhere near the strength/bulk to be able to play the PF position.
For a SF he's got tremendous length, is a good rebounding, and a good shooter.
If you could swing a deal with Detroit to pick him up cheaply, I'd certainly take a flier on him.
A couple years back I thought he was one of the best prospects at Summer League provided that he could bulk up, but he gets no playing time behind Prince which has certainly hurt his development. This year he's again showing a lot at Summer League, so he'd be the guy I'd try to take a cheap flier on.
As to Thornton, the good thing with him is that you don't have to deal with another team, you could just sign him, as I don't think he's signed to a team right now. But I'd rather play TWill at the SF than Thornton if I had to choose between the two.