I'll be short and direct.
How much should be payed an artist for a single sound track?
Of course it depend on many factors first of all their curricula, who they are.
So to be more precise or to make a comparison, take Ilaria Graziano (she cooperated with Yoko Kanno for some of Ghost in the Shell (and many more, which included Napple Tale for Dreamcast) lyrics..) as a standard...
I forgot to mention, I'll prefer to be answered through PM...