It's certainly a term used by chefs, but not much by the general public. It would work as a site built by a professional chef for chefs...
I can't in all honesty give you a value purely it comes down to an individual wanting to use it. I think your guide can only be down to any traffic it receives. It doesn't need to be high figure to have value, purely because these are going to be professional chefs looking for 'Chefing' related topics. It's a real shame it's not the .com.
Sorry to add, just checked. And it seems the Cheffing spelling may be the preferential spelling. either way value for this term in .Org is only going to come if it has traffic