JSFiddle - React, Tailwind, and code Playground

HTML

<p>Количество:</p>

<div class="number">
  <input id="count" type="text" value="1">
  <span id="plus" class="plus">+</span>
  <span id="minus" class="minus">-</span>
</div>

<p>Цена:</p>
<div class="price"><span id="price">2 500</span>руб.</div>

JavaScript

var COST 	= 2500;
var COUNT = 1;

var countField = document.getElementById('count');
var plusBtn 	 = document.getElementById('plus');
var minusBtn 	 = document.getElementById('minus');
var priceEl 	 = document.getElementById('price');

plusBtn.addEventListener('click', increaseCount);
minusBtn.addEventListener('click', reduceCount);

function bindPriceToElement() {
	var newPrice = calculatePrice();
	priceEl.innerText = newPrice;
}

function bindCountToInput() {
	countField.value = COUNT;
  bindPriceToElement();
}

function calculatePrice() {
	return COST * COUNT;
}

function increaseCount() {
	COUNT++;
  bindCountToInput();
}

function reduceCount() {
	COUNT--;
  bindCountToInput();
}