You are getting some good input here. Another issue with burl is that it is bad about cracking as it dries. There are ways to slow the drying process to minimize the cracking (drying it in the dark in a bucket of sand or wrapped in newspaper), but there is no guarantee even then. If the tree currently has leaves on it, the moisture content is going to be higher and the chances for cracking increased.
I would make your friend aware of all of these factors before agreeing on any price.
It's kind of like baseball cards. The only value they have is what the next guy is willing to pay. Let him see how much money he makes off of it sitting in the back yard for awhile and maybe the price will go down.
