Most restaurant chefs don't cook with actual Cognac, but with a similar French brandy. When I went surveying chefs I knew, I found that DeVille French brandy was the choice of many, as it is very reasonably priced, and similar in style to the pricier Cognac. It's not bad for sipping, either, fwiw!
I think you'll find that Armagnac is just as pricy as Cognac, and much more robust in flavor. That might be what you're looking for, but if you're hoping to save a couple $$, I don't think it will.
Most chefs don't cook with real Champagne, either. They use one of the Cremants or other sparkling wines that cost significantly less.