Not sure what they gave up to get him yet, but I believe Houston got a 4th round pick for him before the 2009 season, and that was after he played in 6 games for Houston in 2008. Last season, he didn't see any regular season action, so I can't imagine the Giants gave over anything better than a 6th round pick for him.
I like this signing. Once he learns the playbook, it's puts an established vet QB behind Eli, should something happen to him. Not that I think Sage is taking us to the Promised Land, but he can be a nice spot-starter. The only downside is that it probably means we'll be keeping 3 QBs on the final roster, and there are holes on the team that need filling.
EDIT: Just read that the Giants also got Darius Reynaud in the trade with Minnesota. That definitely addresses our PR/KR needs. Looks like the Giants parted with a 5th round pick in 2011 and a conditional pick in 2012.