I like to sell mine between 500-600 lbs. At that weight, it's still cheap to put weight on them, and you only get a few cents less per pound than for 400-500 lb. calves. This way, it seems to maximize my initial investment in the price of the calf, and expense of the milk-replacer. I come out making between $175 and $250 per head profit (the difference within that range is determined by what I had to pay for the calf and the market price when I sell him).
Alot of people say that you maximize your profit when you sell between 350-500 lbs.- and that would be true if you figure it only on cost of feeding grain and hay- but when you add in the initial investment of the price of the calf and the milk replacer, I feel that those latter expenses are better ammortized when you sell 'em in the 500's.