Well, I am not sure if it's moderately priced, but you might want to call Locke-Ober about the prices for their lunch
If the prices are in your range, you would have a hard time finding a lovelier room and sense of Boston.
ANd don't forget that Chinatown is just off the southeast corner of the Common.
FYI - it's the Common. no S.
Anyway, I mention this to help you avoid getting weird or uncomprehending looks when you ask for directions. The parking garage under the Common is one of the more reasonably priced ones in that area, btw.
Depends on what you want to eat.
Grotto is just down the street from here towards Cambridge Street and serves a nice Italian sit-down lunch. Sam LaGrassa's is not far away going towards Downtown Crossing and offers very good traditional deli, while Chacarero does their unusual but excellent Chilean sandwich thing pretty much right next door.