adding a formula to items in a drop down list
 

Hello,
I am wondering if anyone can help adding a formula to a text cell of a drop
down list. I understand how to generate a drop down list, however I am stuck
on the formula.
This is the best way i can explain it:

The text is the drop down list might be
- a pinch
- a hand full
- a shovel full
- a wheel barrow full

Each amount has a formula with the amount in ounces or pounds that would
auto sum in a total column. I hope I explained i properly. Any help would be
greatly appreciated.

PS I know I can do it by adding another row but I would like it included in
the down list. Thanks.