Hmmmm... I think when most people (that I know anyway) refer to "Finish" it implies the calf has been fed-out and is ready for slaughter. IF you "grain finished" a calf you would almost certainly loose money if you took it to the sale barn since you had probably a couple of hundred bucks of feed in it and people don't go to salebarns to look for freezer beef. If you "grass finished" it then it would just be another cow at the sale. But in that case you probably would have made a higher "percentage" profit if you had sold it at a lighter weight. The USDA budget calls for (and feedlots typically like ) the target Dun mentioned of 1100-1300lbs. So grass finishing one to that range narrows your customer base (and bid opportunities) at the sale. So like Txshowmom implied, what are you realy trying to do?
If you want to pasture weaned calves for awhile and then take them to the sale barn then 600-700 lbs is a good target weight. This is sometimes refered to as "backgrounding" a calf. This is the weight that the USDA budget calls for (and feedlots typically like) to start calves on feed at. If you start with weaned calves in the 350-450 lb range you can get them to 700lbs in 5-6 months depending on your pasture and the calf of course.