It is no small matter to toss the equivalent of many hundreds of thousands of dollars away. The wonder is that many people did it anyway.
If it takes fifty $20.00 gold pieces to equal a thousand dollars, and if gold is currently ~$1,200/ounce, then if a slave cost $1,000.00 in the money of 1860, that makes the manumission of each one equal to $60,000.00 in today's dollars.
The moral hazard here is the belief that human life can be equivalent to money. Again, this is a consequence of the Muslim religion, not the Christian one.
Another interesting case of an apologist who rationalizes irrelevancies and ignores duty and law. It wasn't Lee's job to moralize over the terms of George Custis's will - it was to execute it.