I know the proce would very depending on complexity and decorations etc but can someone give me an idea of a starting point for a basic gumpaste heel and shoebox cake price?

First one would need to know the size you plan on making.

cake is approx 9x13 and covered in fondant cream cheese buttercream and chocolate cake.

I usually have a set price per slice for specialty cakes that is much more than buttercream or fondant. So take the serving of a 9 x 13 and times it by your specialty price and there you have it. Extra gumpaste work such as flowers and details could be extra.