What is the usual fee for a person who will sing at the ceremony?

They can range all over the place, depending on who is singing, the part of the country, how many songs, if they are friends or professional singers, if its a small wedding in a small church or a grand affair in a cathedral, etc. I've seen them range from $40 to $1,000.

Often times the ceremony site will have suggested guidelines, so you may want to check with them too and if the singer is a professional, he/she will tell you the fee.