svileff,
<!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>
$(function() {
function numberWithCommas(str) {
return (+str).toFixed(2).replace(/(\d+)(\.\d+)?/g, function (c, b, a) {
return b.replace(/(\d)(?=(\d{3})+$)/g, "$1 ") + a
});
}
$(".t-descr_sm").each(function(i, el) {
var num = (el.textContent.replace(/[^0-9.]/g, "") || 0) * 0.8;
var txt = numberWithCommas(num);
$(el).after(`<div class="t706__product-amount-sale">${txt} р.</div>`)
})
});
</script>
</head>
<body>
<div class="t706__cartwin-products">
<div class="t706__product" data-cart-product-i="0">
<div class="t706__product-amount t-descr t-descr_sm">1 210 р.</div>
</div>
<div class="t706__product" data-cart-product-i="1">
<div class="t706__product-amount t-descr t-descr_sm">2 110 р.</div>
</div>
<div class="t706__product" data-cart-product-i="2">
<div class="t706__product-amount t-descr t-descr_sm">567 р.</div>
</div>
</div>
</body>
</html>