Determine number in a field based on word-choice in a drop-down menu

I'm trying to insert a number in a field based on a word I've chosen in a drop-down menu.

Example:

I have a drop-down menu named "Fruits". In that menu is "Orange", "Apple" and "Banana".

I have a different field called "Price". If the user selects "Orange" in the "Fruits" drop-down, the value of "Price" is 10. If the user selects "Apple" the value of "Price" is 15. If the user selects "Banana" the value of "Price" is 20.

Been working with this for awhile, and feel like Im very close, but can't get it to work. If someone could help me I'd be grateful.

Thanks,

T.