Ask HN: How much to charge for strategizing a checkout page?

I am new to design and have been given the task of revising checkout pages. Especially making a four step checkout into compact one page checkout to increase conversions. Here is a blueprint I created - https://www.dropbox.com/s/cvb0gy6fpxzuje6/one-page-checkout.jpg?dl=0

What do you think about the worth of this blueprint. I know its not the entire design but a ballpark would also help. I am not sure how much to charge.