Most economical way to keep slave population up on a large scale operation?

Would it be more cost effective to buy more slaves to replace the aging ones or have your current slaves reproduce. Would raising slave children be worth it because even though they are technically free when they are firstborn they can’t work and must be fed and cared for. Also you run the risk of your slave mothers dying in child birth potentially costing you a slave. Also your slave baby might die making all that for nothing. Buying new slaves skips all these problems but every time you need new slaves it will cost you money and you could potentially miss out on some of their value if you don’t buy them young.