Yeah, but ...
Edit:
when I'm not certain, I just google it! :p
"sont cher" yields 4,880,000 hits and
"sont chers" yields 2,560,000 hits.
Both are evidently common, but one much more popular than the other.
Yeah, but .....
How many hits did "Sony & Cher" have ?!?!?!?!? :p