How to calculate the costs?

2) This one should be easy, but I guess not easy enough for me.  I have a field where a specific quantity will be entered, i.e. "Number of copies" , and that number will be multiplied by a per-page price, which might be changed from time to time.

I have inspected your form and I saw that you have already created calculation that is multiplying "Number of printed pages" and "0.25".

 Is 0.25 price per page? If the price per page will be changed, you will need to do that manually inside of Form Calculation field.

I did not found field "Number of copies" inside of your form. You can add that field to your form and include it inside of calculation, if you want to calculate total price based on number of pages, number of copies and page per price. 

How to Perform Calculation in the Form?

Here is my demo form: https://form.jotform.com/62362992380966 

Feel free to test it and clone it.

If I did not understood your question correctly, please explain in more details what you are trying to achieve. 

Hope this will help. Let us know if you need further assistance.