I have no idea what you should charge, but I don't think SamTherapy is far off the mark.

This sounds like a highly ambitious and time consuming project and there will probably be mega reworking of the art once the guy sees it and decides he wants changes according to what he envisions in his head.

Two reasons he doesn't want to give you a price:

1. He honestly has no idea and hopes you do;

2. He knows the work involved and has gotten price quotes from others and is hoping that you being "inexperienced" will undercut the price and he can get it done really cheap.

I do recommend a solid contract drawn up by a lawyer that addresses reworking of images beyond a certain point.  You will find that while you create the art based on vague concept input given to you based on their ideas, things change rapidly once they have something in front of them.  Suddenly they get all kinds of ideas and you soon find yourself doing rework galore to add things they suddenly decide that they want or remove things that they suddenly feel they don't like etc.

I remember I was doing a website for someone and what started out as a 2 or 3 work project turned into 4 and 5 and 6 weeks. It was never ending!  I finally put my foot down and told them that I had delivered the goods and did extra work at no cost to them,  and that I had gone way above the call of duty and if they wanted any further changes, alterations, additions etc they would have to pay me more.

"It is good to see ourselves as others see us. Try as we may, we are never
able to know ourselves fully as we are, especially the evil side of us.
This we can do only if we are not angry with our critics but will take in good
heart whatever they might have to say." - Ghandi