Pricing for iPhone app screen designs

Hi all,
I’ve been contacted to design for an iphone app screens but I don’t have any idea how much to quote the client. Most of my projects are web designs. This is the first time I’m designing for iPhone. Can you give me an approximate figure?

Thanks,
Luc

My daughter has done a lot of I-Phone app designs - however, all of her deals have been for a percentage of sales.