Adding a value to the selected value of a dropdown

Google Sheets doesn't include this as a built-in option, but you might use Google Apps Script that is already integrated with Google Sheets.

Example

Below, there is a JavaScript function using the reserved name onEdit. This simple trigger is activated when the spreadsheet is edited by a user using the Google Sheers web app or one of the mobile apps. Please read Extending Google Google Sheets - Google Apps Script and Simple Triggers - Google Apps Script.

1. Create a new spreadsheet

2. On A1, insert a dropdown. The Data Validation will be opened.

3. Replace the default values for the options with numbers

4. Click Advanced, and in the section If data is invalid, select Show a warning.

5. Click Done and close the Data Validations side panel.

6. On B1, add the number to us to multiply the selected value.

7. Click Extensions > Apps Script.

8. Set the Apps Script project title.

9. Replace the default function with the below function.

   function onEdit(e){
      const multiplierRange = e.range.getSheet().getRange('B1');
      const multiplierValue = multiplierRange.getValue();
      e.range.setValue(multiplierValue * e.value);
   }
   

10. Click the Save button.

11. Go back to the spreadsheet.

12. To test, select a value from the dropdown and wait a bit. The A1 will show the selected value, and a few seconds later, the value will be replaced with the calculated value. A1 will row a red triangle in the upper right corner because the entered value is invalid, unless it matches a value among the dropdown options.

Once you have a clear understanding on how this works, adapt it to meet your needs.