I bought this pair of capris second-hand for less than $5 a year or two ago. They had the button when I bought them. It was silver color 4 hole button. As you can see the button fell off.
I removed the threads.
I went with a silver-colored jeans button for the replacement.(because there were silver buttons on the flaps on the back pockets) I poked the bottom through from the back and pushed the button over the shaft.
Since the pack of buttons cost around $1 I still have around $5 invested in this pair of capris.