Why are you better at mobile website design? I mean, don't you use html? If so it shouldn't be too hard for you to adapt to "full websites", right?
Anyway, I recently have been building mostly responsive designs, which scale to the device a visitor is useing. See a sample by resizing your browser here:
http://designbyjanja.com/JewelryDesigner
I think, you should invest the time to learn (check out lynda.com) something like that, because I'm not sure how much money businesses are willing to spend on a mobile site. I currently charge around $550 because I think that is the most somebody would spend for it.
When I started my business, I checked prices of compeditors in my area and went with the cheapest pricing until I had enough work to keep me busy.
I guess the theory is: if the work gets more then your time, that's when you raise your prices.