boh... io farei così...

codice:
<!doctype html>
<html>
<head>
<meta http-equiv="Content-type" content="text/html; charset=iso-8859-1" />
<meta http-equiv="Content-Language" content="it" />
<title>Esempio per il Forum di HTML.it</title>
<script type="text/javascript">
	function fissaValore() {
		document.miaForm.importo.setUserData("sOldValue", parseFloat(document.miaForm.importo.value).toFixed(2), null);
	}

	function controllaValore() {
		if (isFinite(document.miaForm.importo.value)) { fissaValore(); }
		document.miaForm.importo.value = document.miaForm.importo.getUserData("sOldValue");
	}
</script>
</head>

<body onload="fissaValore();">

<form name="miaForm">
<input type="text" name="importo" onblur="controllaValore();" value="0.00" />
</form>

</body>
</html>