JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" name="a" id="input">
<input type="button" value="Посчитать" id="check">
<div class="result"></div>

JavaScript

const countUp = document.querySelector('#check');
const input = document.querySelector('#input');
const result = document.querySelector('.result');

countUp.addEventListener('click', () => {
  let value = input.value.replace(/\D+/g, "");
  if (!value) {
    input.value = '';
    alert('Только цифры');
    return false;
  }
  let sum = +value * 0.04;
  let res = sum < 400 ? 400 : sum;
  res = sum > 20000 ? +value * 0.03 : res;
  result.innerHTML = res;
})