Like e.g. there are two select options type components that has various amounts of $ to select from. So if user selects in one field $100 and $200 then the next field below it would generate the total which would be $300 in this example.
I did research, and the module Webform Calculation Components does the exact thing I want, but it does that only with numeric fields where user has to type amounts manually. So it's a no-go. In my case user has to select from drop-down list already given amounts.