GUI Design fees

does anyone know what are avergae fees for designing a graphic interface for a software (on freelance basis and maybe as part of the staff also)?

The program sells in a range of $50-100

The job may involve just redesigning current icons and buttons but it may grow to a full interface redesign.

It would more than likely involve just concept developemnt and creating of actual graphics but would not probably go into mechanics of the programming (I mean using any type of program that actually creates the interface on the programming level) Just to compare though if you could throw in this number also, that would've been great.