Predetermined Sliding Scale

I am trying to calculate user costs for a number of projects. These projects will pay for their users on a sliding scale. As the number of users goes up, the unit cost per users, goes down.

I have a chart of costs that is based on two factors. The number of users and which application they are using.
I have blocks of users from 0-10,11-20,21-30,31-40 ..... The Second factor is which application they are using. For simplicity they are using APPs A or B, OR A+B. There is be a different cost based on which application and how many using it.
an ASCII chart has been provided

# users    | App A   | App B  |  App A+B|
0-10       |$ 5/user | $3/User| $7/User |
11-20      | $4      | $2     | $6      |
21-30      | $3      | $1     | $4      |
31-40      | $2      | $1     | $3      |

Project A|uses App A  |10 Users|=$50 USER Cost
Project B|Uses App B  |15 Users|=$30 USER Cost 
Procect C|Uses App A+B|32 Users|=$96 USER Cost

I would like some help in creating a formula for user cost that will factor in which app they use and how may users on the system. Can someone help point me in the right direction?

I have an intermediate understanding of formulas and functions in excel.

Thank You!