Budget based on project amount

Budget based on project amount

Hi all,
i have a problem with combination of Budget type=  project amount  and billing method= based on staff hours.

If i enable this combination of budget oprions, in my project all the fields related to cost are not filled


If i change the billing method based to Based on project hours, it works. 

But in this case i have costs caluclated on fixed costs and not on the single user cost and for me is totally uselfulness

How can i get the budget cost but calculated with rate ?

Thanks