<script src="http://code.jquery.com/jquery-1.8.3.js"></script>
$(document).ready(function(){	
	$.ajax({			
		type: 'POST',
		url: 'ajax.php',
		data: 'id_tarifa=45',
		success: function(msg){	
			alert(msg);
		}
	});	
	
})

<script>
function NumCheck(e, field) {
	key = e.keyCode ? e.keyCode : e.which 
	
	// backspace 		
	if (key == 8) return true 
	// 0-9 
	if (key > 47 && key < 58) { 
		if (field.value == "") return true 
		//return field.value.toFixed(2);
		regexp = /.[0-9]{3}$/ 
		return !(regexp.test(field.value)) 
	} 
	// . 
	if (key == 46) { 
		if (field.value == "") return false
		regexp = /^[0-9]+$/ 
		return regexp.test(field.value) 
	} 
	// other key return false 
	return false;
	
}
</script>
<input type="text" onkeypress="return NumCheck(event, this)"/>

	-ms-transform:scaleY(1.2); /* IE */
	-moz-transform:scaleY(1.2); /* Firefox */
	-webkit-transform:scaleY(1.2); /* Safari and Chrome */
	-o-transform:scaleY(1.2);/* Opera */