I bet 7-10m if Roma are reasonable. To be fair, I wouldn't accept 5m for him if I were Roma. I just don't know if a big club like Roma would let one of their starting 11 go on loan-to-buy move. It's not like he is a castaway at a giant club, or a superstar at a small club.
*edit- Funny that I've always rated him very highly, but it never seems he's gotten the attention/praise that Clint Dempsey, Landon Donovan, Tim Howard, and a few other Americans have. I think he's always been one of the best players in a USMNT shirt.