"What does a kilogram of sugar cost?"
Translation:Que vaut un kilogramme de sucre ?
I think "combien coûte un kilo de sucre" should be accepted - am I right?
Yep, exactly. it'd even be a more literal translation. Except that I am not sure 'kilo' should be accepted as a written answer, even if perfectly valid for a spoken one. "Combien vaut" works as well.