Calves should be weaned at around 80kg and eating 1.5kg meal per day over say a week.
The balancing act is to determine if the weaned calves have been weaned correctly and the additional cost over those on milk is worthwhile. In theory when they are weaned you should should be starting to get a better idea as to how they are shaping up - so you may have a better chance of avoiding the poor performing calves. Conversely, you are paying someone else to get them that far and then they take their cut out of the cake too. There is also an extra movement on the calf's passport.
With calves on milk, you can be sure they are weaned them correctly. There is also a better choice of stock. You take the risk that you have pulled in a poor one as there is no real way to tell how they'll turn out at such a young age. We always seem to have at least one that is stunted for one reason or another. When you get them on the meal you need to keep them going and not be holding them back. We are setup for feeding milk and its the way we have always done it. Buying in on milk is a gamble, as you have no real indicators of performance and with their young age they are more susceptible to pneumonia and other ailments.
Writing it down makes you realise that weaned is the better option, but on milk is how we buy, but I'm not sure if that will continue in the future. Depends on what breed & sex of calves that you want and how long you are willing to wait to get them.
We have a buyer who we get them from as we are PT and in no position to have an agreement with a diary farm - due to small numbers.