If it is really that big an anvil it could be worth as much as a $1000. That big of an anvil would be 3 foot long. There where a few that big but most where in the 150-250 lbs range.
Also the condition makes a big a bog difference. The horn needs to be in good smooth condition with a as sharp of point as possible. I don't mean a sticking sharp point but down to an 1/2 inch or so.
The condition of the square sides along the top are important. You can't make a good square angle on an anvil that had broken corners.
Do you have any of the anvil attachments??? Like the cut off. hole punch,or small round anvil head? These are getting real hard to find as they get lost and many do not know what they are.
Please post a picture. I have been looking for a real good long anvil. Mine are the shorter 250 lbs everyday working anvils. The real long heavy ones usually where not used everyday. They took up too much room around the forge.
Also where are you at??? I am in North-East Iowa.