Populate hidden field with results of JavaScript formula

I've attached a basic form. The form has two date picker fields (State date and End date), only the first one is mandatory. Users can pick a start date and an end date. What you need to do: The hidden field needs to be automatically populated with a cost based on this formula: The first 3 days are $15 each, every day after that is only $10. Some examples: Example 1: If the user entered the start date 02-02-2016 and the end date 04-02-2016 (that's 3 days) then the hidden field would be populated with the value $45. Example 2: If the user entered the start date 02-02-2016 and the end date 06-02-2016 (that's 5 days) then the hidden field would be populated with the value $65. Example 3: If the user entered the start date 02-02-2016 and the end date 02-02-2016 (that's 1 day) then the hidden field would be populated with the value $15. You get the idea. Some things to also bear in mind: 1) The users might accidentally choose a wrong date, and then try again to pick the date they meant to choose initially. So the hidden field value needs to update automatically if the user is picking different dates. 2) Because the end date field is not mandatory, the user can just enter a start date if they want. When this happens (when the end date field is empty), the hidden field value needs to be the words "Cost Unknown".