Notice: Undefined index: value in C:\wamp\www\corti\presupuestos\precio.php on line 10
var co = $("#codart").val();
var pag = 'precio.php?value='+co;
this is because
$_POST can't read url strings query. and you are sending your variable in url string query.
that's why you have to use
$_GET in your case.
Otherwise, you need to use
ajax such as mentioned in many websites articles to perform post.
and here's one for example: