Ask your local flooring dealers.

The "fuming" process, if it's the classic process, involves some nasty smelling chemicals. And has to be done after sanding.

Or do they mean "a stain that mimics fumed oak"?

I am getting fumed white oak flooring installed in my new home. The fuming will be done off site before installation. Our lumber guys have large metal boxes that they use to fume the wood. Once the fumed white oak is installed, then we are finishing it with clear Rubio Monocoat. We are getting random width 4,5,6 and 7 inches boards and it is $5.50/foot plus $1.00/foot for the fuming. My installer is charging a little over $2.00/foot and I am applying the Rubio Monocoat myself. From what I understand, Rubio Monocoat makes a product that will make your floors look fumed, but results are hit or miss. Fuming onsite is really dangerous and isn't recommended. The chemicals needed for fuming are very noxious and you wouldn't want to be exposed to them. My lumber guys just put the boards in their big metal box, lay a tray of high concentration amonia in the bottom and the next day it is done. Nobody is exposed to the chemicals and the wood is absolutely gorgeous!

There is no substitute for a real gas fumed wood product. No liquid fume will replicate the look and depth of the reaction of the ammonia and the tannins found in white oak.

You may not be allowed to fume oak onsite in some locations because it is hazardous. The process needs to be in a closed contained environment. Fumed oak furniture was traditionally done in fuming tents.

You'll need to ask a better question if you want better answers.
Solid wood or engineered?
Prefinished or site finished?
Pattern?

How can one ask for pricing comparisons with such scant information?