How to calculate total cost based on number of nights with weekly discount

How would I calculate the cost ($35/night) based on total number of nights, which is fixed until the 7th night, and then there's a discount. There is an additional $10 discount after every 7 nights... so for example, 3 nights would be $105, 9 nights would be $305, and 15 nights would be $505.

I would also like to add in other fields to this number, but not sure how to do it from a drop down menu with many different choices.

Any help would be greatly appreciated!

This is possible with calculation features at JotForm.

Here is an example:

- Total number of nights added with a Number field.

- Two Form Calculation fields to implement the calculation.

- Calculated number of weeks is calculated with a floor calculation function. 

- Total calculation formula.

Result:

https://form.jotform.com/62626271671962

You can clone the form to your account to check the back end.

If you would like to add some value from the dropdown.

- Add a dropdown field.

- Assign Calculation Value to the DropDown Options.

- Include your dropdown to the calculation formula.

If you need any further assistance, please let us know.