Somehow they are the same, but as Muhanad said: هذا سعرنا الأدنى: When a salesman tells you this, it means: this is the lowest price for this item in our store. So he's referring to the price of one of their items. هذا أدنى سعر: If a salesman says this, he means: this is the lowest price of this time any where, and not just in their own store. Bottom line; they mean the same but used in different contexts! I advise you not to use "هذا سعرنا الأدنى", so if you want to refer to something that you own, you better say: هذا هو السعر الأدنى لدينا.

They have the same meaning but the speaker is different هذا سعرنا الأدنى Means. This is our lowest price So you are talking as salesman هذا أدنى سعر This is the lowest price So you are just pointing out that the price of this item is low