|Is the data (e.g. 100) static or variable?|
If it's static, you can use an IF statement:
Let's say your dropdown is in E1. In E2, enter:
However, I'm guessing your list is more extensive than 3 items and that the data may change, so I doubt you want to hardcode the formula.
I'd set up a table like:
1 jeans 100
2 sweater 50
3 t-shirt 25
and then use VLOOKUP:
Obviously you'll want to populate the dropdown directly from the range where the items are to ensure an exact match for the VLOOKUP function.