Dinner in Cohasset
Depending on your definition of "affordable," I would recommend the Square Cafe in Hingham (double-check that they're open on Sundays). They have a "cafe menu" with options ranging from $11-$14 and it's less than a 10 minute drive to the Circus.
Not sure what your idea of affordable is, but not too far from the Music Circus in Hull is "Saporito's Cafe", just off the main drag at Nantasket Beach.
Matter of fact, I think that's where we're heading for dinner on Sunday.
John in Norwood