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!

Sample files often help here so people can see where the data is to located etc...

Assuming (as implied) the calculation is not marginal then in reality it's basic INDEX/MATCH (or even SUMIF with INDEX) - attached is a basic ex.

I've added a tiered/marginal approach also just in case...

edit: ignore the marginal - won't work accurately with the same bands as used for the non-marginal approach (would be 0,10,20,30)

Ok, so INDEX and MATCH functions are what I need to look-into
Sample uploaded to original post.

Yes ... your values in Col A should be the value of the lower boundary (only) eg:

0
10
40
80
150
230
350
500
750

Then it's just a case of matching as appropriate, eg:

=$F17*INDEX($B$3:$D$11,MATCH($F17,$A$3:$A$11),MATCH($B17,$B$1:$D$1,0))
copied down

Note: your sample value for G18 is wrong I think - should be using D4 rather than D3 as multiple

Yes ... your values in Col A should be the value of the lower boundary (only) eg:

So in my range of users, only the bottom of a range, say 0-9, only use the 0, That makes sense, it will check if value is between it and the next in range.

I just printed out your formulas to look at it on paper.
I am using your formula, now we shall see.

Thank you so so much for your quick and useful replies

So I took your formula, and augmented it to fit my spread sheet.

=$F17*INDEX($B$3:$D$11,MATCH($F17,$A$3:$A$11),MATCH($B17,$B$1:$D$1,0))
F17 is # of users to calculate against.
I get that INDEX is table of Costs
1st MATCH # of users in calculation, then range of available user blocks(represented by the bottom of the user range)
2nd Match is matching the project APP type against the user price headings

right?

But it seems to result in" #NA "in the cell