i know standard 18" cv8 alloys with tyre weigh just over 19kg so i would have thought that a 19" alloy would weigh more, just to give you a rough idea. After market alloys can easily weigh 5kg less a corner
From picking the std 19' alloys up got to say they do weigh a fair old chunk, would say easily over 20kg's but not sure how much of that weight is the tyre.