Why won’t my function capture the value of my input element?

I’m working on a freeCodeCamp Javascript project and I currently need to obtain the value given by the user on my input element. I’m practically certain I have my code correct, yet when testing it on freeCodeCamp or in a Code Pen I created the value keeps returning as undefined.

let cash = document.getElementById(“cash”).value;

const purchaseBtn = document.getElementById(“purchase-btn”);

purchaseBtn.addEventListener(“click”, () => {console.log(cash)});