It depends on the doe and if you can sell milk, cheese, soap, meat, manure....lot of people don't have sales in them.
For the first several weeks the kid will drink alot of what she makes, how much is she milking? If she is a poor milker than milk sales may not be as much as it takes to feed, hay, worm and care for her. A good nubian doe can easily have enough milk for a kid, (who I would bottle feed and not let drink as much as he wants off her) and give you at $7 per gallon, at least $3 or $4 in profit each day. This doesn't sound like alot but at costs being at about 1$ a doe per day here, having each doe earn her keep and give you $3 each per day, that's over $900 a year in milk sales just with one doe who milks for 10 months. Why you only keep kids who will improve your sales, and her buck kid will not, unregistered he is likely not going to fetch even the price at weaning that he has drank in milk. If you can get him on the bottle and sell him quickly because he is spotted or extra cute, than sell him now. Learn to make cheese or soap and your profit per day from your milk will increase. It may also be illegal in your area like it is mine to do all the stuff we do, you have to deal with that aspect also, but milk customers are really loyal folks.
If she is healthy is key, if she is a good milker who will stand to be milked and easily milk out a couple of quarts a day, 3 is better, 4 is best, than snap her up. IF she is a poor milker, if she is sickly, really underweight, swollen knees, if her udder when you mlk her isn't butter soft when empty, than pass. Nobody wants to purchase milk from you out of nasty looking goats...alot of milksales is about
PR. Vicki