If it is solid I would say yeah that is reasonable. It would be a good deal. But, burl sometimes can be pretty airy, the waste factor could be big. I don't know walnut burl that well just looking at that pix I would 30-40% of it could be scrap or small use piece, or you could cut it and resin those outside pieces.
I would have him cut it open before you pay. If dried and cast in resin, you can make multiple times that amount selling blanks. Or sell a couple to recoup your investment.
Look on etsy what burl blanks are selling for.