Alina777,
<!DOCTYPE HTML>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
</head>
<body>
<script type="text/javascript">
function calc() {
var pop = document.getElementById("pop");
var tup = document.getElementById("tup");
var result = document.getElementById("result");
var price_pop = 0;
var price_tup = 0;
var price = 0;
price_pop += parseInt(pop.options[pop.selectedIndex].value);
price_pop > 6500 ? (tup.checked = false, tup.disabled = true) : tup.removeAttribute('disabled');
price_tup += (tup.checked == true) ? parseInt(tup.value) : 0;
price=price_pop + price_tup;
result.innerHTML = price;
}
</script>
<select onchange="calc()" id="pop">
<option value="0">Значение 1</option>
<option value="5500">Значение 2</option>
<option value="6500">Значение 3</option>
<option value="7000">Значение 4</option>
<option value="7500">Значение 5</option>
</select>
<input type="checkbox" onchange="calc()" value="1500" id="tup" />
<br><br>
ИТОГО:<span id="result">0</span> руб.
</body>
</html>