Hi Ryan,
This is very possible.
There are several good examples that come with LC Designer. When you click new, select "based on a template". Have a look at the purchase order template.
In your dropdowns go to the Object / Binding tab and select "specify values", check that these match your dropdowns, 1, 2, 3,... etc. Then you can refer to the dropdowns in your calculations. In javascript:
subtotal.rawValue = price.rawValue * quantity.rawValue // the dropdown is named quantity
Good luck,
Niall