More generally - if they don't sell it at a restaurant supply store (or a store catering toward the restaurant business), you are going to be overpaying for it. This is true for almost everything you really need in the kitchen.
Deeper carbon steel pans are great for stir frying, but I have yet to find another use for very shallow carbon steel skillets.
Note that Lodge makes skillets that it calls "carbon steel" which are actually just especially thin cast iron, and I love these. They are thick enough to perform like cast iron, but much lighter. I have 10 and 12 inch skillets (Lodge model numbers CRS10 and CRS12), and they're my go-to skillets for almost everything: pancakes, omelets and fried eggs, sauteing, and shallow frying. The only thing I don't use cast iron for is scrambled eggs.
EDIT:
Nope, nevermind, I'm wrong. I just checked my pans, and my Lodge thin cast iron skillets are the same thickness as those Mineral B pans -- 2.5mm. My crepe pan is thinner. So you're right, those De Buyer pans probably are excellent replacements for cast iron -- lighter, cheaper, similar performance.
EDIT 2:
Damn you batbomb, now I have to go shopping for pans. Restaurant supply store you say? Maybe ebay?
This only really matters for searing and shallow frying, where temperatures need to stay high and stable. For lower temperature activities like sauteing, evenness of heat is more important. Delicate things like crepes and egg dishes are sort of special cases, requiring both evenness and a surface that releases the food without sticking.