You can use vlookup with 1.
Make 1 table
A | B | |
---|---|---|
1 | 30 | $15 |
2 | 51 | $13.75 |
3 | 101 | $13 |
4 | 301 | $12.50 |
Now if you have data like this
C | D | |
---|---|---|
1 | Quantity of Shirts | Cost |
2 | 40 | =vlookup(c2,$a$1:$b$4,2,1) |
3 | 103 | =vlookup(c3,$a$1:$b$4,2,1) |
You will get the result.