JSFiddle - React, Tailwind, and code Playground

HTML

<p>Копії: <input name="count_copy" type="number" id="copy" min="1" max="25" placeholder="1" value="1" /></p>
    <p>К-сть сторінок на листі: 
        <select id="paper_select" name="paper_select">
           <option value="1">1</option>
           <option value="2">2</option>
           <option value="4">4</option>
           <option value="6">6</option>
        </select></p>
    <p>Результат: <span id="result"></span> грн.</p>

JavaScript

stock_price = 17;
element_copy = document.getElementById("copy");
element_papers = document.getElementById("paper_select");
element_result = document.getElementById('result');
function calculate() {
    var copy = element_copy.value;
    var copies_on_paper = Number(element_papers.value);
    var price = stock_price * copy / copies_on_paper;
    element_result.innerHTML = price;
}
element_papers.oninput = calculate;
element_copy.oninput = calculate;
calculate();