FormatCurrency(Expression[,NumDigitsAfterDecimal [,IncludeLeadingDigit [,UseParensForNegativeNumbers [,GroupDigits]]]])
Something in this neighborhood:
<form name="myForm">
<input type="text" name="myTxtBox" onBlur="MoneyFormat()">
</form>
<script language="vbscript">
Sub MoneyFormat()
myVar=formatcurrency(myForm.myTxtBox.value,2)
myForm.myTxtBox.value=myVar
End Sub
</script>