Depends about the ability of the artist,the fee of the render is about your effort and time,there is a way to sell your render,that i thinks probally should work in your in this case,"charge for days" only you know what is the price about your time(T) and effort(E) and convert this in the time and effort for "1" day,put a value for 1 day,if you say that this render scena have been made in just 3 hours,then divide the (T) and (F) of the day in the time that you have to work to produce this scena,your could add more variables to the ecuation,taxes,transport,etc,etc,this is just a sugerence,works for me!