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();
var next = $(el).next(".product-cart__number");
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.price_1_20;
if (num >= 20) price = data.price_20_50;
if (num >= 50) price = data.price_50_100;
if (num >= 100) price = data.price_100_500;
if (num >= 500) price = data.price_500_1000;
if (num >= 1000) price = data.price_more_1000;
pr.text(num * price)
})
})
});
</script>
</head>
<body>
<div class="product-cart__price">Цена:
<span class="price-val" data-price_1_20="890" data-price_20_50="821" data-price_50_100="751" data-price_100_500="709" data-price_500_1000="695" data-price_more_1000="688">890</span>
<span class="ruble"> руб.</span>
</div>
<div class="product-cart__number number">
<span class="minus">–</span>
<input class="count" type="text" value="1">
<span class="plus">+</span>
</div>
</body>
</html>