Well, the $1000 in 1860, considering inflation, today is roughly equal to $30,910.

Ultimatelly the exact amount of money isn't important. What if they offer you 30 million or 30 billion dollars. Let's say that most of the people if tempted with such an opportunity, will have their breaking point. Then the price of a friendship could be calculated as the difference between the market price of an animal, and the price at which the owner sold them.