Probably because it has syntax errors
if ($("#chbShoutcastV2").is(':checked')) {
var v2 = document.getElementById('lblPrice').innerHTML;
var cena = "399";
var suma2 = Number(v2) + Number(cena);
document.getElementById("lblPrice").innerHTML = suma2;
}
else {
var v2 = document.getElementById('lblPrice').innerHTML;
var cena = "499";
var suma2 = Number(v2) + Number(cena);
document.getElementById("lblPrice").innerHTML = suma2;
}