Create and populate a cell if checked

I have a budgeting table with earnings and outcomes, so for the monthly payments (like subscription etc.) I’d live to have that at a certain date a cell duplicate itself. I was thinking to use a checkbox so if it’s checked than it duplicate periodically, but I can’t figure out the right formula. Can you help me?

by duplicate, do you mean multiply? For instance if there is a monthly charge beginning on October 1st of $15, the cell should show $30 on Nov 1, $45 on Dec 1st, etc.?

To do the above you will need a defined start date and a formula a little something like this:
IF({Checkmark Field}, {Charge Amount Field} * DATETIME_DIFF(TODAY(),{Start Date},'months'),0)