Stay_Strong,
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">
</style>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.12.4/jquery.min.js"></script>
<script>
$(function() {
$(".product-cart__price").each(function(indx, el) {
var pr = $(".price-val", el);
var data = pr.data("price");
var next = $(".product-cart__number",el);
var count = $(".count", next)[0];
var num = +count.value;
next.on("click", ".minus, .plus", function() {
num += $(this).is(".minus") ? -1 : 1;
num < 0 && (num = 0);
count.value = num;
var price = data[0];
if (num >= 20) price = data[1];
if (num >= 50) price = data[2];
if (num >= 100) price = data[3];
if (num >= 500) price = data[4];
if (num >= 1000) price = data[5];
pr.text(num * price)
})
})
});
</script>
</head>
<body>
<div class="product-cart__price">Цена:
<span class="price-val" data-price="[890,821,751,709,695,688]">890</span>
<div class="product-cart__note">
<span>В зависимости от количества цена может изменяться</span>
</div>
<div class="product-cart__number number"><span class="minus">–</span>
<input class="count" type="text" value="1"><span class="plus">+</span>
</div>
<div class="product-cart__buy"> <span>Заказать</span></div>
</div>
</body>
</html>