<html>
<head>
<script type="text/javascript">
function SetPrice(a) {
var e = document.getElementById('price');
e.innerHTML = +e.innerHTML + a * +document.getElementById('x').value;
};
</script>
</head>
<body>
<span id="price">
100
</span>
$
<input type="text" id="x" value="10" />
<input type="button" value="+" onclick="SetPrice(1);" />
<input type="button" value="-" onclick="SetPrice(-1);" />
</body>
</html>