I don't know where to start.
There is so much that happened in 1862 and the years before and after.
I will post a few key points.
The Acts and Proclamations that lead up to the Emancipation Proclamation are the key to this "price per head".
First Confiscation Act, Second Confiscation Act, Supplemental Act, etc.


http://www.presidency.ucsb.edu/ws/index.php?pid=29502
In Lincoln's State of the Union address December 3, 1861
He first mentioned the possibility of buying freedom with federal taxes.

$300 per enslaved person in compensation from the government.
This bill freed over 3,000 slaves, expending more than one million dollars in federal payments.
http://www.presidency.ucsb.edu/ws/index.php?pid=70146
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http://en.wikipedia.org/wiki/Emancipation_Proclamation
In July 1862, Congress passed and Lincoln signed the "Second Confiscation Act", containing provisions intended to liberate slaves held by "rebels",[32] but Lincoln took the position that Congress lacked power to free slaves within the borders of the states unless Lincoln as commander in chief deemed it a proper military measure,[33] and that Lincoln would soon do.


This is when Lincoln brought Admiralty into the picture. He knew the feds lack jurisdiction so he had to use his military position to enforce his goal, under Article II, section 2 of the United States Constitution.
This military action is well documented.

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